If they have come up under FCA UK then you can make a complaint to the Financial Ombudsman.
You need to have had
There is a process that you need to go through although you can phone them, you will need to put everything in writing
you have to have exhausted their complaints procedure before referring matters to the ombudsman.
They are not entitled to keep back any fees if they didn’t say they would charge those fees.
If somebody does work for someone else, provides a service, or supplies goods, and does so as a business to a consumer, then under the Consumer Rights Act 2015 the:
Goods must be as described (not applicable in your case but please read on)
Of satisfactory quality
Fit for purpose
And any service must be as described and carried out with reasonable care and skill. It would appear that they failed to carry out the job with reasonable care and skill otherwise you wouldn’t have lost the investment.
If no price is specified then the price must be “reasonable” and if no timescale is specified the timescale must be “reasonable”
If on the other hand goods are being sold by a private individual to a private individual, they only have to be as described.
In the case of a vehicle it also must be roadworthy otherwise it’s a breach of section 75 of the Road Traffic act.
The latter two points not applicable in your case worth mentioning for completeness if ever anything else crops up with regard to Consumer purchases or services.