I can now answer my own question (in the hope that it will help someone else). Here's what I did.
I went on the Glow-Worm web site and navigated to the list of installation and service manuals for discontinued boilers. The one for 70FF is listed but the pdf file is missing from the site. However the 60FF manual is available and the two models are similar enough for that one to be of use, so I downloaded it.
By following the flowchart given in diagram 9.2 of that manual I was able to determine that the PCB had a fault. I found a local plumbers' merchant who could supply a suitable PCB for £98.63 inc VAT.
Changing the PCB was a doddle. There is a terminal block attached to its cover - that has to be temporarily removed. All the wires that need soldering into the PCB come with it, already soldered in, so it was simply a case of disconnecting the other wires, removing the old PCB, putting the new one in its place, connecting everything up again and putting the terminal block back. There is a wiring diagram inside the bottom of the case which helps no end.
One complication - there are two different PCBs used in the 70FF. The one with a single fuse is the right one for my boiler, and appears to be identical with the 60FF PCB. Had it been the one with two fuses I might have been struggling, since I do not know that the fault diagnosis flowchart is the same. I was lucky - someone else might not be.
Still, that said, I now have a working boiler again, and I am looking forward to a hot bath!
Tony, January 2009
