First of all, this isn’t a Police matter as it’s a civil matter it’s not criminal.
There is nothing in law that prevents someone from recording another person, the problem more comes from what you do with that recording for example if you share it with third parties without their permission they could take civil action against you.
I would say that recording another tradesman (because they say it in public) is very risky and in my view is morally wrong. You’re using his opinion against someone else in his industry which could affect his business and his reputation and so you could end up with action against you. You’re also using a workman and not paying him which is just costing him money because you’re not willing to spend the money.
A secret recording (even if it was illegal) can be admissible in court however this is at the discretion of the sheriff.