Just in case someone else want's to know.
I have found two captive screws on the top of the boiler, one each side. They're on the outside of the case and I didn't find them previously because I can't easily see the top because it's in a cupboard.
The idea is that you undo all four and then slide the cover forwards to remove. Fine, but it's not done yet. I'll need a ridiculously long screwdriver to clear the cupboard to undo them and I don't have such a thing despite having a mass of tools. Will have to cobble up an extension somehow.
dorneyman
March 2021