By instructing a barrister or solicitor to review your refund claim you will receive independent and objective advice about whether there is a reasonable basis to make the claim. That way you should be confident that if you make the claim, HMRC are likely to agree with it even if they open a check into the claim. However, even some solicitors firms operate claims refund companies that work on success fees and you need to be sceptical of them because of the commercial incentive for them to make speculative claims.