Geoff - here's the one I made - using a single TIP120 Darlington transistor and a 555 timer:
http://www.backyard-astro.com/equipm...dewheater.html
Works wonderfully.
Here's my finished product and a couple of images of that website. All I needed to buy was a good old etching kit from Jaycar and away I went. Mine drives the primary heater, one for the secondary (if there is one), the finder scope as well as the eyepiece - all off the one parallel heater circuit.