The solicitor should have given you an engagement letter when you instructed him. This should have stated the fee basis, and also the complaints procedure you could follow if dissatisfied. If you did not get such a letter its absence is a ground for complaint.
If you did get one check what it says about the fee basis. If it is clear that it would be a % of valuation there is probably nothing you can do. Otherwise, you can go through the firm's complaints procedure and then to the independent solicitors complaints body (look at Law Society website). Alternatively, you can pay some of the fee (I think it is half) and ask for the fee then to be checked independently - again the Law Society site should give info.
Incidentally, I'm surprised that the fee is based on a % anyway - I thought that basis of charging went out years ago as it is so unfair and does not reflect the work involved.