Yes, there is a reasonableness limit, and this is especially true in consumer transactions.
If you were given an estimate and the final bill is a lot more than
what you were expecting, you can dispute it.
The final price should be ‘reasonable’. The law doesn’t say what
counts as reasonable, so you’ll have to agree it between you. You
- the estimate you agreed to [if there was one]
- any changes, and why they happened
- anything that happened that was beyond the control of the trader,
like bad weather or the cost of materials going up
When it comes to work itself, the act states that a tradesman or
professional has a 'duty of care' towards you and your property. Any
standard or price you agree must be honoured. But if it isn't agreed
in advance the work must be done to a reasonable standard, at a
reasonable cost, and within a reasonable time.
So if you haven't fixed a price, you don't have to pay a ridiculously
high bill. All you have to pay is what you consider 'reasonable' and
invite them to sue you for the rest. Be careful though, in some
circumstances when you are withholding payment you may have a claim
made against you by a supplier if you are in breach of contract.
What's a reasonable amount would be what similar tradesmen would have
charged for the job. So get a few quotations.
NB that article refers to The Supply of Goods & Services Act 1982, which was partially superseded by the Consumer Rights Act 2015.