Hi Hammers.
Who were you working for? The householder or the builder?
In either case, I would recommend normal credit control processes - polite but firm request in writing with deadline (EVERYTHING from now on in writing - no phone calls and no texts) followed by prompt court action.
It would be sensible to say that you won't register the boiler or complete the benchmark book without payment, (in fact, I would definitely put it in writing, otherwise the lack will be used in the future to make you look incompetent). I would recommend against doing anything else in your capacity as a GSR.
The rights and responsibilities of Gas Safe Registered engineers are all about SAFETY, not credit control. Just imagine the uproar if (say) a policeman or doctor was making use of their official powers in order to enforce payment of a debt? It doesn't matter how right you are, you will be wrong if you abuse that privilege. Its not worth risking your ticket for it.