Ok. What is is it you want to achieve?
The loss of profit?
Or just to defend the £2500?
If there was defective work then you should have been given an opportunity to put it right - this is mitigation of loss.
Therefore the customer can not just demand £2500 back.
If you refuse all he can do is issue a claim in the County Court.
You can dispute the work is of poor standard and that he has failed to mitigate his loss.
As such in any event the value of £2500 would be a small claim so even if it went to Court, you both would not need a lawyer.
Can I clarify anything for you about this today please?