So to get this right.
Your not in any Scheme, but you’ve been working for this builder for some time, testing & certifying another electricians (sparks 1) work, who can’t prove his credentials to carry out electrical work.
For some reason the builder then decides to get a Part P spark (sparks 2) to test & certify this particular job (installed by sparks 1). Sparks 2 then invoices the builder, for carrying out some sort of test & certification, on this job. The builder then asks you to pay sparks 2 bill? (have you received any monies, for this particular job?).
If that’s the case, I wouldn’t have anything more to do with the builder, not pay this invoice, go and speak to Citizens Advice Bureau.
I can’t see the builder has a case, as this arrangement you two have been following is not one of the methods for notification, as prescribed in Part P.