Hi neil, did you get this sorted? I've exactly the same problem and happens virtually everytime I go for a drive now. As soon as I park up and leave it for a short period it just won't start again. I recently found out that if I sit and feck about turning the key back and forward, let the immobiliser kick in and out it normally fires randomly after a few mins and I'm off. I'm guessing it's an electrical problem but before I spend a fortune on an auto-electrician I'm looking at changing the ignition switch behind the barrel, there's a HOW-TO guide in the knowledge section of this forum.
Let me know if you had any joy and would appreciate anyone elses advice as I'm looking to sell the car soon :( want it sorted for the new owner!
Ta