I am not a lawyer (although I did take a course on contract law
).
Anyway, your rights depend on whether you were using the accountant to provide a service to you as a consumer, in which case the
Consumer Rights Act 2015 applies. If you employed the accountant in a B2B transaction, you'd apply the rules in the
Supply and Good of Services Act 1982. It sounds like it would likely be the latter as it was in regards to your taxes for your business.
Either way, your rights are the same, the service must be of satisfactory quality and fit for purpose. It sounds like this is neither of those. In your next chat with them, mention the legislation and see if they agree to file any new forms free of charge.
If they are unmoved, you could mention court, but it might prove difficult. You might be able to claim for any extra fees and interest you pay have pay the IRS for late filings or such. However, if you would have had to pay for different forms at the time of the original (if the accountants had done the filing properly) then it would be reasonable to pay for them now. Also, if you would have had to pay your UK accountant extra to run the numbers for a U.S. tax year as opposed to the U.K tax year, then you probably wouldn't be able to get compensation for that either. But that's just my very basic understanding, so could be wrong.