Contractual dispute with builders

I’m looking for an experienced solicitor who would help me win a legal battle with builders.
I’m in a dispute with a building company over unauthorised extras that I refused to pay. We failed to reach an agreement and they simply walked away, leaving the job unfinished. They didn’t agree to mediation, so I have no choice but to take this matter to court. I want to make a claim for cost of materials paid but not delivered, damage caused to neighbouring property that I have to repair, interest.
I need a legal advice on my position in this dispute, help drafting a letter of claim, support with the court application. The amount I will be claiming is £25K so I need to keep legal costs proportionate to the claim, rather than an open chequebook. I'm based in London