Customers are becoming more aware, not necessarily of the need/requirement to use a gas safe registered engineer. More the instal is complete and then they ask for the building compliance certificate, then the installer disappears, and then a registered business is contacted to "register" the boiler.
As a registered guy you have to consider lots of different potential issues, did the customer fit it and are they just trying it on? Were they fully aware of the illegal guy, happy to save a few quid and now want it signing off for whatever reason; or were they just naive.
My advice to the customer would be, request a safety check from gas safe register, it's free. Once completed a full list of all installation defects will be issued and then the genuine registered guy can look at what is needed to put it right and charge accordingly.
I used to charge a flat fee of ÂŁ1000.00 , do you think that unreasonable? I would remove the flue and take the boiler off the wall, re flush the system (because it was probably never done in the first place). Test all the pipe work and refit, then you become responsible for the whole system, gas, hot and cold, central heating etc.
Doesn't sound too much now does it.
The alternative, maybe there is a safety issue and you signed it off for ÂŁ50.00, causes an incident. HSE get involved, you go to court and the starting point is 6 months in prison and everything that implies. Or maybe you get lucky, but why would you not charge in line with your skills and experience? No one is lucky forever, and customers have zero loyalty when it comes to dropping you in it!