Let me throw a scenario at you Brendan, and I'm sorry that it doesn't involve mortgages (seeing as you'd seem to prefer to discuss that!)...
Lets say the contract says 3 months notice, but no-one in the place is ever actually asked to work in the full notice period. So you've worked there a few years and seen other people in the same or similar roles come and go at 4 - 6 weeks' notice without difficulty... so you, for whatever reason, have an offer elsewhere and think it's safe to assume that 4 - 6 weeks notice shouldn't be a problem based on everything you've ever seen happening in reality in that workplace, and based on the fact that you know your job isn't one which would take more than a couple of weeks to fill.
But then, for purely personal reasons, the MD decides that they are going to make your life difficult and they throw in your face a contract that says you're legally obliged to give 3 months notice - you say ah hold on now, haven't I been a good employee and often worked above, beyond and outside of the terms of what you contracted me to do...?! (Sunny's posts refer) And they sneer and say, "I don't care, you signed the contract so we're holding you to it!".
In that circumstance, if it were you, can you honestly say that having exhausted the negotiation route, you'd sit there and serve out the notice period?! And bear in mind that having given the notice, you're going to be out of a job in 3 months, and if the other employer won't hold that position, you're completely goosed, unemployed and without entitlement to JSB for 9(?) weeks...
I have to say Brendan if your answer is yes, then you're a true martyr to contract law.
Although, at least on the good side, when you're unemployed and upsidedown on your mortgage, you can go to AAM to get good advice on going bankrupt in the UK...