Jonathan,
the link to the switch is : http://uk.rs-online.com/web/p/push-b...tches/7082967/ Its a bit expensive but I chose it to take a bit of punisment.

I checked the switch a few times with my digital vernier and repeatability is within 0.3mm so its not too bad. I had also looked at making a voltage touch probe like Jazzcnc is talking about, that might be version 2 if I can get my hands on some solid clear perspex square bar or similar. The spindle mount is 25mm thick alu so its stronger than it looks but I have started on making another two to replace that one to strengthen it up.
The head comes down quickly until 20mm above the estimated tool block, depending on the guess value you input into the tool database, then slows down to touch speed.

Ian