Not seen an answer to this question so thought others might find it useful: I spoke to Nationwide about the "0% green additional borrowing" feature and they said that DIY is not allowed. The way it works is that you get a quote for something that you want to do from a company give it to them and they pay the company directly. So you never see any of the money.

Might be useful for some but I'm going to save money by doing it myself as the prices these companies charge is extortionate.