Final update: they won.
I had to back down and sign the contract. They would not take no for an answer.
I took legal advice from our family solicitor. He basically said, don't go there, only the legal profession will win. If I wanted to prove contructive dismissal, the onus is on the employee. If I refused to sign the contract, and somehow forced them to pay me what I am due, then I would win the battle but lose the war (terrible way to think of your job, isn't it!). He said they would prob be waiting in the long grass for any excuse to fire me in the future.
So I signed it (there have been other issues regarding holiday accrual and so on that I have basically had to compromise (ie capitulate) hugely on as well).
It's a bit depressing to think that they have so much power over me, depsite all the employment law to the contrary.
However, pregnancy has gone well so far thank God, despite the incredible stress I have felt due to this issue.
Just wanted to thank everyone for their input, it really helped me get my head clear. Hope these posts help someone else, even if it is not good news really.
Thanks again.