There are 2 versions of PCB, I think one has pump connections on and the other doesn't. So you would need to buy like for like. And as for replacing the PCB yourself, you probably would be able to do it as it's not a major job.....but to acces the PCB you would need to remove the case which...
Sounds like it's not flame rectifying, which would probably be either electrodes or PCB. As you have had electrodes already I would go for PCB, but also the leads to the electrodes would need to be checked over.
You should be ok to use the spray cans. I have had a similar issue before and found that the out buildings water supply was isolated by turning the neighbours mains water off, could be a shared service?
0.0143 PT @ 16.4 MB (MAX)
0.0003 PT @ 10.5 MB (AS FOUND)
Looks like on max BP ratio is through the roof, but when BP set as it was found (Medium) the ratio is ok.
I doubt that the fault you describe has anything to do with the combustion seal. If it's perished then yes it needs replacing, but I think your fault will still remain a problem.
The temperature lights on the front are not the temp of your hot water, that's the heating side (which also heats your hot water) are you sure it's not a 105 instant? You said it has a preheat, if it has a preheat then its a 105 instant . If so then the hot water is factory set, the temperature...
The existing cylinder probably will have a coil, or it could be Primatic. I'm not 100% sure but I think you can have primatic fully pumped but the pump may need to be fitted different way. (I may be corrected) but if it was primatic I would probably change it.