Have we overpaid stamp duty because the house has an annexe?

A company claims we have overpaid £6,208 but we are wary of pursuing thisQ I’ve received three letters from a company claiming to be specialists in reclaiming stamp duty advising us that we may have overpaid £6,208 in stamp duty when we bought our house because it has an annexe. They have offered to make a claim on our behalf on a no-win, no-fee basis (I’m not sure what the fee would be but assume it would be a percentage of the successful claim).We purchased our house in