Iain, it is standard practice to switch the positive and this is what I have done on the two installations I have done.
Remember that on the 12v the +ve should be brown, so it is advisable to check with a mult meter.
I'm assuming you want to match up your new switch to the other switches and sockets, so have a look on the Clearcut Conversions website. They are based in Barnstaple, Devon.
Colin
Edit, senior moment here in Greece brought on by high temperature and possibly alcohol. The 12v +ve should be BLUE, as picked up on below by Alan.