No CH with Potterton Puma 80?

I have a problem with a Potterton Puma 80 in that the CH does not appear to be working and there is no heat going to the radiators. The hot water works fine and stays hot if run for a while and the boiler fires on demand OK
I had the CH set to timed and when switched to constant, the boiler just does not fire on demand.
We had problems a couple of months ago with the boiler not coming on at all and find there was a loose connection on the PCB board which has now been resoldered and everything has been working fine.
The pump was replaced in November 2004. I queried the sludge with the plumber, as I am in a very hard water area and get a lot of problems with limescale generally. The plumber said we should have had an inhibitor fitted. This may be irrelevant though to this problem.

The main problem I have is that I am unemployed and cannot afford to pay someone a lot of money ( I live in London ) to come and get this fixed, however I am freezing. I am therefore hoping it's something that will be fairly easy to fix and I can do myself. I don't know much about electronics other than the basics, however I'm fairly technically minded.

Please help

Nicola