Hi there,
When you say you see a slight current, do you mean voltage and what are you using to measure this? Some testers are very sensitive and will pick up the slightest voltage, like some neon screwdrivers and volt sticks. Cables can appear to be live when they are in fact not, it could be that the cables are running alongside other cables which are inducing a small amount of voltage onto the cable in question. The other thing to consider is extract fans, mirror heater pads etc that may be on the same circuit. It could also be that you have a permanent live at the fitting, which then goes to the light switch and then back to the fitting. Could you tell me how many cables/terminals you have at the light fitting?
Basically it is difficult to know exactly what is going on with your system without seeing it. If you have access to a proper voltmeter then I would suggest you test between L-N, L-E, N-E. Ideally you should get 0V between any of them but you may get 2-3V if it is induced by surrounding cables.
Hope that helps...

Thanks CWatters and Zambezi
I was using a neon screwdriver and a little 2 prong affair of a similar nature - both showed distinct glows when "off", maybe a little less than a quarter of the brightness of when "on". I'll get a proper voltmeter on it and see what that says.
There doesn't appear to be a fan wired on the same circuit, but there IS a permanently on live feed at the fixture, which goes down to the switch and back, so it seems likely that the phantom current is being induced (if induced is the word I want) along the way.
Thanks for your help. I feel less clueless now.