How can I fix my Potterton Puma 80?

My combi boiler has broken down, After lighting the pilot the system fires up and hot water is produced for about 5 minutes max, then it cuts out. As soon as the central heating is switched on the boiler cuts out. How can I mend it, does anyone have up-to-date, clear instructions I could follow, the manual we have is too complicated to follow. Please help, have been recently made redundant and cannot afford call out fees.