Potterton puma 80 dies after a minute or two?

My eleven year old puma has ceased to suply both hw and ch,
when either is required it starts up nicely but cuts out after a minute or two usually taking out the pilot as well,
I would like to attempt repairing it myself as my finances are a little thin right now and cant realy afford to be taken advantage of by local cowboys.
So any advice on this problem would be gratefully received.
Also I realy need a diagram but the potterton page advised in other posts does not have this perticular model listed,
Any ideas on where i can download one would be great.