There's no written contract, we just started
The job is running on a quote, a few emails and goodwill, and something has now gone wrong.
What's happening?
A huge amount of construction work starts this way: a price agreed on the phone, a start date, and everyone too busy to paper it. The moment payment slows or the scope is argued, both sides discover very different memories of what was agreed, and the side that owes money usually remembers the smaller version.
Two things are worth knowing before assuming the worst. First, a contract need not be a signed document: the quote, the emails, the messages and the conduct of both sides can add up to one, and establishing what they add up to is a document exercise, not a shouting match.
Second, the Construction Act applies to construction contracts whether or not they are in writing, so on contracts it covers, the payment and adjudication rights it provides are generally available, and a set of statutory fallback terms, the Scheme for Construction Contracts, may fill the gaps the paperwork left.
The starting point is gathering what does exist, in writing, before memories harden further.
The solution
Establish what was actually agreed from the documents that do exist.