Claim to HMRC for fees

I have a case that I have had to chase up with HMRC for well over a year which culminated in a formal complaint.  We have now more or less got the case settled and I am looking at the time I have spent chasing up with HMRC issuing the complaint, reporting to the client on the lack of response etc and it will amount to fees of around £600.  I have already indicated to HMRC that I will be submitting a claim for fees and know that the client has to pay them first before HMRC will repay anything.  I just wondered if anyone has had any experience of this and the likelihood of my client getting the fees repaid.  I just want to forewarn my client so that I am not setting him up for disappointment if HMRC have a standard amount that they repay.

Many thanks.