I know what you're talking about. I never had to use it myself, but there is something in the law alright that protects pregnant women - it's called something along the lines of 'pregnancy risk assessment'. I only know about it because my husband had a routine health&safety course in work a while back and they mentioned it there. Firstly, to my knowledge, HR is obliged by law to provide you information on it (it is not restricted to HSE or even public sector). I'll wait till he comes home from work and see if I can get more details for you.
ETA - Just spoke to him, he said that your employer is legally obligated to give you an official risk assessment. It's forms you will have to fill and sign, so it becomes an official document in your file. If they don't do it themselves, approach HR about it. Essentially, they have to reevaluate the conditions and environment in which you work and see if anything needs to be changed to accommodate the pregnancy. If there's any medical issues, they must take it into consideration also. Remember that it is not one-sided - i.e. their decision is not final until you are (within reason) satisfied. They are supposed to improve on your conditions based on your circumstances. That's all he knows, hope it helps.