I have a Potterton Netaheat Electronic EP3000 gas boiler (about 18 years old). The problem started about three months ago, seems to be getting worse and is as follows. When I switch the boiler (heating and water) onto constant for the first time (after it has been off overnight for example) the fan comes on, the pilot ignites and the boiler burns as it should. After a while the burner turns off (as it should). Then when the pilot is supposed to reignite to recommence burning it fails to do so. The ignition spark can be seen to be working (through the window), but the pilot isn't working - the fan works and the ignition spark clicks repeatedly, but nothing ignites. If I turn the boiler off for a while (maybe 2-3 hours) and then turn it on again, the pilot ignites as it should (and the problem proceeds again as described above). So the problem seems to occur after the boiler has been running for a while. Any suggestions would be appreciated.