[MatthewH]

Thanks, I'll look into the cam timing kits. Might be worth buying the tools as the belt will need to be replaced at some point either way!

I've been speaking to another A8 owner in USA who had the same symptoms as me - it turned out to be the primary fuel pump in his case. I did another fuel pressure test yesterday and the pressure does drop from ~51psi to ~46psi around 30 seconds after startup. The secondary pump masks the issue for the first few seconds as fuel pressure appears normal, but when it turns off the pressure drops. I believe the pressure should remain constant even when the secondary pump turns off.

I've purchased another fuel pump (£160) so I'll get that fitted in the next week and update this thread with the results.