I appear to be the only member of my team to work their contracted hours. All the rest go way over and I'm starting to feel like they (and my boss) resent me. What should I do?

Wondering why on earth it says nine-to-five in your contract when you seem to be expected to be there till all hours will feel familiar to many, especially in the City. A long-hours culture is seen as the norm in many roles and environments - not in your contract, but a fact of your life, whether or not it's productive (and physically present doesn't necessarily translate into physical profits, as we know). Does it really need to be that way? And what's the legal position on this, anyway?