Do I have any right to receive an invoice and is there anything further I can do to get one?

Mistral001

If you have paid for thr boiler, then surely it is a receipt that you need. An invoice is for an amount that is due and since there is nothing due, an invoice cannot be sent.

As others have said, this could be VAT problem that the installer thinks he has. However that is none of your business and not relevant if you are private individual and cannot reclaim VAT. It should not be of any relevance to the insurance company either.