If you'd told me what is happening (you've had a go at putting a light in and it won't switch off) I would have said you've wired the live loop directly to the fitting without having a switch in the circuit.
If it is wired correctly as you say it is, the only other thing could be the switch not breaking contact, but you say this is unlikely.
Are you hoping that I'll say the fitting is faulty? If the fitting was faulty it wouldn't come on at all, a fault isn't going to generate electricity to make it not switch off.
Adam
May 2009