It's not a lot of money?
You had free storage of your Deeds for several years. Presumably, you were instructing another firm to handle the sale. The solicitor might have waived the fee if they were getting the sale.
And, at the very least, the solicitor had to check that they did, in fact, have the Deeds and had to arrange to schedule them and have receipts typed.
Most solicitors, contrary to the commonly held belief that they are bloodsucking parasites, are actually very bad at charging properly for routine matters like this.
mf