Dave,
Chris' first recommendation is excellent - I bought it recently and is already proving invaluable.
I have a couple of others to add:
Desks by Andy Charron (Taunton pub) which has plans for 7 desks.
Fine woodworking on Tables and Desks (again by Taunton) - 32 extracts from FWW magazine. Not plans per se but good sources for ideas if a little old by now maybe?
Both should be available from a variety of UK sources.
Cheers
Tim