We have the same problem as Baz but with a Potterton profile boiler?

The pilot light comes on but goes off again. The radiatiors start to get warm and then go cold again. We have aired them and replaced a few of them due to leaks, but we thought this would solve the problem with the boiler. They are still not working. We don't know why the pilot light won't stay on. We even changed the timer switch but we still have the same problem. Can anyone help? It feels as though there is a problem with the boiler itself, but can't see where to re-ignite the pilot light. Is it possible? Thanks for your help!