Hi Richard, it looks like the British Mac keyboard is different from the British Windows keyboard.
On the Mac, the '\' key is near the return key. On Windows the '\' is to the left of the 'z' key.
To type '\' in Windows try hitting the key on the left of the 'z' key. If this works you can add a key mapping (Preferences -> Keybaord & Mouse) to correct this behavior.
Also, make sure you have the same keyboard layout selected in Windows and on your Mac. For example, if you're using British on the Mac but you're using US English in Windows then things can get messed up.
Thanks,
Sailesh