Work trying to start my maternity early.

Hi All- I would really appreciate some advice please! I haev been off work with SPD for the last couple of months. I am aware that if I am off sick in the last 4 weeks of pregnancy they can bring my maternity forward. Howvever as I work 3 days per week and have 15 days holiday left (equiv 5 weeks) my doc signed me off till then and then I planned to take holidays. My boss is now saying that they are planning to start my maternity early. This would imply that I will have to return to work a month earlier than planned and lose these holidays as they do not have to pay them. Is it me making a fool of myself or are they in the wrong??? They are buggers and I had a real battle with them when I requested flexible working and I am not convinced they atr actually that au fait with the legal side of it. Any advice please?
xxx