I have a lot of troubles with my chain jumping of the front chainrings. And generally slapping about like a wet fish.

On closer inspection the chain has a LOT of lateral flex, which my mates inform me is bad. I'm all set to order a new one, but want to ensure my cassette is not going to need replacing at the same time.

So,
How do I tell if my cassette is 'worn' ?

i would almost certainly bet you will need a new cassette too

tragically is probably right you know, and maybe a middle ring too.. You cant really tell by eye if the cassete has had it.

order the chain and cassette as a matter of course, and see how it runs on the middle ring.

First off, I'd be checking to see if the chain is too long or if the front mech is set properly. Lateral flex shouldn't be an issue.

Get a chain checker and test your chain. If your chain is worn, your cassette and probably the chainrings will be too, which means it's time to replace the lot! 🙁

I guess you can do it without buying the tool too. Just put a new chain on, and if it skips/slips/jumps under pressure, the cassette is worn. Wear on the chainrings should be visibly obvious, but you'll get the same slipping under pressure with a new chain.

If you want to avoid the same thing happening again, buy three (or more) chains, and swap them over every month or two, so you reduce the stretch, and therefore the wear on the rest of the drivetrain.

Pretty much what Davy says- put the new chain on at the beginning of the week and then go for a spin and see if it works. If it doesn't, order a new cassette. (But if everything is worn to the point where the chain is slipping, it's virtually certain you'll need one).

If it's still rough you might need new chainrings too.

Get yourself a chaintool. Rohloff do a good one. Measure the stretch in your chain after every ride. When the chain tool indicates unnacceptable stetch (0.75-1.0mm), get a new chain. This will save you a fortune on cassettes, which you will only now have to replace at a ratio of about 3 cassettes to 1 chain.

Although the bike has been with me for a while, it's not used as much as it should be. (Monthly rather than weekly).

When in a Big ring to big ring combo, the rear mech is pulled forward a fair bit (more than 90deg to the chainstay). So length is fine.

It's a full sus early Trance, so it's a maestro suspension setup, which means it should not be having problems with variable drive length.

Under rough descending with a Big chainring, middle casette combo the thing is flying off both inside and outside the chainrings. 😕

If you want to Check your chain a steel tape measure can be used if you don't have the special chain checker.
You need to do this in old money – inches,and using a steel tape. On a new chain 24 links measure exactly 12 inches. A worn chain should be replaced at 12 and 1/8 inches max. but preferably at 12 and 1/16 inches.

hmm Big and big is not a good combo

Yes, your chain stretch is caused by inefficient ring-to-ring ratios.

Rule of thumb – granny ring to inside first three cassette rings only. Middle ring – full range. Big ring – outside last three rings only. Your big-to-big usage is what's stretching the chain and causing your problems, I think…

he never said he was in big to big, he said big ring and midway on the cassette….. 🙄

As tinsy said, he just said how the mech looks with the chain in big/big, not that he uses it.

the thing is flying off both inside and outside the chainrings.

That's not to do with a worn chain, that's a badly set up front mech!

Never sure about the whole replacing chain regularly thing, I just run my entire transmission into the ground and replace the lot. I'm about to do mine now actually, having used it for almost a year. Using KMC X9 SL chains I'd have needed at least 4 throughout the year to preserve the cassette, now ok, I may not now need a new XTR cassette (and I really do, it is ruined), but I'd have spent £200 on chains!

I agree with the other posters about first just try changing the chain, then the cassette, and finally the rings.

granny ring to inside first three cassette rings only

That seems a bit cautious to me and I'd like to understand the reasoning. I thought you could comfortably go half way down the block when on the granny. Compare that chain angle with the angle you get with front-middle, rear-top, which is apparently OK.

Correct, I don't use Big to Big whilst riding, just to look at the chain length using the rear mech angle.

Riding is Big to middle.
So I can start pedalling when the gravity effect runs low 🙂

My Front mech looks to be setup right, and unless I've caught it, my LBS set it up for me.

Also – I'm not getting the chain skip/slip under strain, it's just the rough terrain that's making it jump so much.

I intend to measure it when I get home, but I am now suspecting the rear mech spring might be too weak ?

90 degrees in big big sounds a little long for the chain. I set mine up so that at the longest part of the stroke, without the mechs, my chain is light in big big plus one pair of links.

the longest part of the stroke, without the mechs

What does that mean Onza ?

On a hardtail, it was always, miss the mech and measure the chain big big plus one pair. With a bouncy toy, it's the same, just make sure you do this in the "longest" part of the travel, ie, where the chainstay length is at it's maximum.

It's only going to be a couple of links different from where you are but it might help. Also, what's the rear mech like? Has the spring in that started to go weak?

Cheers for the explanation. – Understood !

Will investigate this evening.
Rear mech is the OEM Deore unit that came with the bike.
Front is a Deore LX.

sometimes the main spring in a rear mech can get a bit dirty. On some it's possible to take it apart, clean and regrease. That should help. Like njee20 says, a worn chain shouldn't cause the chain to come off when decending.

I find this happens to me sometimes. I do the same and run big ring/middle on the downhills to keep the chain under a bit more tension. I also turn the cranks if/when I get chance on long decents to bring the chain back if it has bounced off.

That's exactly what I do Onza, but it happens more times than not, which makes me think I need to shorten it and check the front mech as others have described.

At Glentress yesterday the chain lept off and made it forwards thru the front mech and down to the crank-bb bolt.

I was trying to pedal at the time, missed a bridge and very nearly landed in the burn/stream as a result !

Happens to my mate Chris a bit as well. The bottom bosses on his seat tube are just in the wrong place for me to be able to set the front mech up at its best.

Rohloff also do a cassette checker – never used one or even seen one but seeing how much cassettes are these days may make sense

Link here

I've got one of those casette checkers. Don't use it very often. Normally the chain checker at 0.075 means the cassette is okay, at 0.1 it means the cassette needs to go with the chain. (you can save them together and then swap them out when the others get bad. By that time, the chain rings will be going so you can just destroy everyhting together.

I've always set my chain length in absolute top gear, with the rear mech vertical. That has always been long enough, and will allow for those mad moments when I forget what I'm doing and finish up in big/big. But as for OPs original problem I think he's due for cassette, chain and a couple of rings. Get steel rings, and at least two chains and swap them regularly, maybe even after every ride.