On cold start-up it seems to run smoothly, but when warm or after a few
minutes, idle seems to be lumpy, as is misfiring on 1 cylinder - it still pulls
away smoothly when warm / hot, apparently  firing on all cylinders, so I don't
think its e.g a weak compression cylinder (cracked piston ring or leaky valve)
its just when warm / hot that it runs as if one cylinder not firing.

The car has been little used for some months, so this might be a clue.

Any suggestions as to what to look for ?

Would it be worth putting some injector cleaner through, or are these symptoms
a bit more sinister, like the injector pump about to die, although I would
guess it would affect all cylinders if this were the case ?

Thanks,

Nick