Chris, i think you need to read the guidelines available on the Gassafe website, particularly this download:
http://---------------.co.uk/pdf/Who can legally work on a gas appliance.pdf
Which says: "If the work required only involves the replacement of a water carrying component e.g. a pump or central heating control valve, housed within the boiler’s decorative casing, the work could be undertaken by anothercompetent tradesperson e.g. plumber/electrician. Providing the work could be undertaken without having to break a combustion chamber seal"
I read that to include electrical components inside the boiler too, that do not invlove breaking any gas seals. There are a number of boilers (WB?) that have a sealed cover, so clearly they should not be worked on except by Gassafe, or competent DIYers, but for the majority of boilers, it is quite legal to change the electrical components without being GS registered.