The lack of a signed contract is a problem alright, what if anything is there in writing about the extras approved versus the others he completed? How do you prove these other extras weren't authorised?
The Building Control (Amendment) Regulations 2014 likely apply to a complete refurb (depending on the scale of works), and if they do, it is illegal to occupy the building prior to the Certificate of Compliance being issued. That said, that does not give the builder any entitlement to prevent your access to your property.