For those that might of been interested it turns out that the problem was
1-PCB
2-Gas valve...had water ingress and was making the burner fluctuate...There is a breather pipe from the valve which connects into the burner chamber which filled with water....there you go
The problem is that it was actually working with the water in it albeit roughly.I t was only the fact that I had got water onto the PCB that I had changed it. Also, its funny how it was working ealier in the day and then this sarvo...no heating. Have had a word with a matey and he seemed to...
Recently we have had some very heavy rain here and unfortunately our guttering failed and water has gushed onto the flue basket. Next day the heating and hot water were playing up. Now checked inside and thought it might just be a sensor as this has gone several times. Replaced that and still...