Service intervals (after the 600 miles first service) are 6,000 miles (or 1 year) and then 12,000 miles (or 2 years).
The 6k (1 yr) service is a "belt adjust" plus the usual valve clearances, oil change, petrol filter change, etc.
The 12k (2 yr) service is "belt change" plus the rest above. It costs about £100 or so more than the 6k service because of the cost of belts. I think there is no extra labour because the belts come off doing the clearances (but I might be wrong there).
There is a very informative article on Sigma website about belts, clearnaces etc. See the Technical section on their site at
http://www.sigmaperformance.com/.
That should give you some reading for the next day or two!
[Edited on 21-7-2005 by HW]