Hello Fooxy
You might like to look at the website:
www.maternityaction.org.uk
and download some of their information sheets, particularly the info sheet on "Pregnancy and Maternity Rights at Work".
I'm not sure whether your employer is entirely within their rights to ask you some of these questions so it would be worth checking.
You could also give Maternity Action a ring on 0845 600 8533 and I'm sure they would be able to advise you and help with the letter if necessary. I have rung them in the past myself and they were very helpful.
Rachel