@^woody^ is spot on. Start from the point that the contract has been broken because the work has not been completed.
The first action may well be to write formally to the builder and state he has broken the contract by not completing the work in the agreed time. That you are still prepared to pay for the total of the work at the agreed price but only once it is completed and signed off by building control, and you will
not be paying any more at all until the works are certified complete. (with no written contract he has no contractual right to stage payments) Give the builder 7 days to respond and 14 days to get back on site if he intends to complete. Also advise him that if he fails to complete, and the cost to complete the work using others exceeds the agreed cost of the contract with him, you will sue him for the difference (you have the right, but most likely in the end you won't because of the hassle, unless the difference is massive and the builder has assets you can go after).
Irrespective of a written contract, there is an obligation on the trader to complete the work with skill and care.
If he fails (as he will), then you can get another builder to quote to complete the work and get it signed off. paying a surveyor is probably good money after bad, because you won't get reimbursed the surveyor fees.
The builder does have the right to be paid for the work done to date, but the onus is on him to prove the quantities and values, not you, and taking in to account the obligation to complete the work to an appropriate standard, work that is defective and cannot pass building control, has no value no matter how much time and material has been spent on it, because you have no obligation to pay for defective goods and services.
In my experience, although you have the right to legally pursue the other builder, the pragmatic approach of cutting your losses and moving forward as quickly and inexpensively as possible is best.