Builders want extra money

Hi,

We are in the process of having an extension to our house. We were quoted £39k which we were happy to accept and work has been coming together well. While they were digging out the drainage I recall they were having some frustrations, I'm not entirely sure what they were. After they had resolved it, the project manager said there would be some extra to pay, but he said not a great deal, which I'd expected to be a few hundred quid.

This week however he's informed me that the total extra would come to £2200, which to me is quite a significant amount. He's not given me an invoice yet for the additional work. What do I need from him, I.e. What do need him to put on the invoice? How can I ensure that these additional costs are genuine and he's not trying to pull one?