How do I stop it happening?

I've got new chainrings, new chain, and I've shortened the chain as much as possible. Eccentric bottom bracket as well, so I've moved the BB forward so the chain has got plenty of room between chainrings and frame.

Running out of ideas.

when you say "I've shortened the chain as much as possible." – why? – i.e. is this a singlespeeder, or you using gears? (if Gears then you don't want it to short)

next question, how many front rings you using, single, double or tripple? – if either the latter 2 then does it such on all rings or just one or 2?

probably single speed as using an eccentric bb

No, geared. Shortened it to give the mech more tension.

And I'm using a double, and only when shifting down.

Can you actually get chain suck on a single speed?

TBH if you've done all that and you're still getting it then all you can do now is take measures to minimise damage when it does occur like zipties on the chainstay.

chain suck bible

You don't mention the cassette. Has that been changed as well?

Its brand new everything, its a new bike.

Did you buy it complete? Can you take it back to where you got it from?

No and no.

Sorry to hijack this, but i'm inteterestd to know not having had any chainsuck on my previous bikes (luckily), how much damage to the frame does it do? Does it make it weaker no matter what the material steel, alum or ti?

I had real issues with chain suck on my hustler. There wasn't much space between the granny and the chainstay so it was really easy to stick in there and cause damage. It happened with dirty drivetrain as soon as it was even slightly worn. Tried everything as a protector but the chain just cut through it. That was zipties, reinforced tyres etc etc. Here's what I've done and it's totally eliminated it:
1. Dropped to a 2 ring set up (22, 34 at the moment but will change to 22, 36 when that wears out).
2. Took care to ensure that my chain was at just the right length.
3. Bought a E13 DRS, you know the dual ring one with the wee guide wheel and bash gaurd.

Like I say, totally gone. Did all three things at once so can't isolate the cause I'm afraid but it seems the only thing you haven't tried is the DRS. A bit costly so weight up any other suggestions before you splash the cash.

Yeah, I've looked into the DRS, but dont want to have to get that as I hear it weighs 1lb, which is quite a bit of weight to add to a bike. I might first try an anti chain suck device, or just a chaain device without bashguard first.