You file under three circumstances:
1) You owe the UK significant sums of money. If you owe, you ought to pay, even if they haven't asked you formally to file. They don't have a set figure for this, but a balance due of £100 is definitely enough to file. I recommend my clients file if the balance due is £25 or more, for safety's sake.
2) The UK owes you a significant refund. If they owe you, you ought to make a claim to get your money back, even if they have told you "you don't need to file". There's no set figure for this either, but ideally your refund should exceed whatever you're paying someone to claim it for you.
3) They ask you to file. If they send you a Self Assessment form, you must file, even if there is a zero balance due and a zero refund.
Under the new remittance laws, there may be a requirement to file for all nondomciiled persons, which is pretty much everyone who reads this board. We won't know until the laws are finalized, and this law won't apply until the 2008/9 UK tax year.