Bike is an '09 Meta 5.5 with stock kit.

Chain is spotless with no stiff links. Indexing is spot and shifts crisply.

The problem is occurs when pedalling backwards for more than 2 pedals. The chain jumps to the next highest cog. The dérailleur stays where it is (no change in cable tension) and chain slips off the top jockey wheel.

Pedal forwards and it engages in the proper gear.

Its not a problem unless I pedal backwards (doh) which is rare.

I've never experienced it on other bikes I've owned.

I had something similar and found it was the freehub body sticking. New bearings sorted it out. Don't know if that helps.

Knackered jockey wheels can do that too. Try oiling them first – they really do have to be totally shot before you chuck them out entirely.

+1 for knackered jockey wheels

Cheers guys, but I don't think it's either. It's 2.5mths old and it's clean as a whistle and properly lubed.

I was thinking it could have been alignment. The bike had clearly never seen a torque wrench before I went over it myself, maybe a Friday aft bike? Thing is it rides spot on, apart from this.

It's an XT rear hub with sram cassette and shimano chain. Truvativ cranks, x9 dérailleur.

check that the middle chainring is aligned with the middle rear cog. does it always happen regardless what gear you're in? if the chain is 'crossed' then it's probably much more likely

other things which might help –
remove a link from the chain to increase tension
maybe try playing with the b-tension on the rear mech
check the chainring teeth are straight
check the front mech is in the right place
replace entire bike

Alternatively don’t pedal backwards?

+1 knackered jockey wheels (especially given it's an x9 mech)

1) Pedaling backwards = chain likely to derail. End of story. It's worse in the gears at the extreme ends of the cassette.

Practically there's never a reason to do maore than 1/2 a pedal stroke backwards. Only if you need to flick between corners and switch your feet over very quickly, and you're in too high a gear to pedal forwards.

I pedal backwards to line up cranks for ruts, set up for corners, to get the dominant leg to the top of the stroke for technical stuff or any time I need to alter pedal position before applying power. I'd say I often do more than a half stroke backwards…

Anyway, thanks for the input all. Anybody who guessed front mech gets a prize.