Right, done it. The colours I posted earlier are correct. I saw the wiring diagram on MKIV.com, so I knew how it was wired up, but not which colour was what. Anyway, here's the differences:
function pre-facelift colour facelift colour
----------------------------------------------------------------------
+12V White White
+12V no. 2 White+Red White+Red
ACC Pink+Blue Green
IGN1 Black + Yellow Black +Yellow
IGN2 Black + Orange Black + White
STARTER Black + White Black + Red
So, the timer should be wired to white (+12v), black + yellow (IGN1) and black + white (IGN2). It has to be wired to IGN1 and IGN2 (not ACC) otherwise it will run the engine on but not the radiator fan, which could be bad
Just cut a bit of insulation off each cable with a knife, tin it and then wrap the timer wire round and solder it on. I chopped the plug off the timer wire and soldered some extension wires on because I mounted the timer under the stereo in one of those Halfords cubby-hole things.
Mine is the one with a boost gauge built in. Still haven't quite plumbed the tube in yet though.
I'll post some pics later...