Contact the Law Society. My feeling is that you should have been told what the fees were upfront to start with. However, �100 for half an hour's advice is within the bounds of reasonableness.
If this relates to your wife's late aunt's will, you are going to have to seek the advice of counsel on such a complicated issue at some point. Personally, I think you are not going to get much for counsel's advice for 500 quid in these circumstances (you are going to need Chancery counsel who are usually a bit pricier) and 750 is probably closer.
On the upside, if you win, you have a reasonably good chance of getting your costs back either from the other side or from the silly solicitor who mucked things up in the first place.