Did you actually suffer any loss as a result of their negligent services (i'm assuming you must have otherwise the solicitor wouldn't be worried about a neg claim against them)? Presumably you'll still pursue your own action for that loss then?
I argued that the divorce dragged on for 6 months after it should have been settled and that all negotiations after the initial Form E's etc was rendered useless because I was forced to renegotiate in person.
He said I would be asked by the judge to quantify my losses (which I knew), so I listed the time I'd had to spend researching and corresponding with the opposition, my lost earnings, and said I hoped the judge would decide how much of the monies already spent should be brought into question (about £10 grand)...
The crux of their argument was that as I'd managed to achieve pretty much the same outcome as originally proposed, I'd not actually lost out. My argument was that I could have saved a shedload of money if I'd done it myself from the outset!