Hello Vic, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
I would be inclined to put the pay for this work through your company, assuming that the Articles permit such activity. Most companies there days have very wide span of permitted activities. It would also place a buffer between you and your activities should something drastic go amiss. To pay for the actual works I would suggest that you open a separate bank account, he fund it on an imprest basis and you use it to meet the bills.
This work does not attract VAT, but the builder will have to be advised of this fact so he does not put the tax on your neighbour's bills.
I do hope that I have been able to show you a secure and economical way forward in this matter.