There's a rather good guide here: http://www.hendersons.co.uk/template/template1.html
And a place with a broad range of jigs here: http://www.chippyshop.co.uk/ (warning: scantily clad ladies - good old British trade

You can even hire them: http://www.hss.com/g/3166/Worktop-Jig.html
However, I'm dealing with £400+ of worktop here and I seriously don't want to screw it up. I have the old one to have a practice with, but in the end is it really going to save me that much over getting a fitter in for a few hours to do it for me? It's not like it's something I'm going to be doing too often, but it would be nice to know that I can if I have to.
Anyone have any DIY success/horror stories to share?