Hi MF1,
It's not fair, and the glee which many people seem to exhibit when solicitors lose out is a little irritating, but if a solicitor is foolish enough to stipulate a fixed fee without catering for contingencies, then the solicitor, and not the client, should bear the consequences.
My own view is that where a job goes pear-shaped and the agreed flat fee goes nowhere near covering the work done (and there is no prior agreement to increase the fee in the event of things going pear-shaped), the account should show what the full fee would be for the amount of work done. However, if the client refuses to pay more than what was originally agreed, then the solicitor really should take it on the chin (and should also have the good sense to turn away any further work from this client.)