... I think I'm right in thinking that there is nothing my employer can do about it - they just have to pay me the same maternity leave pay as they have just done for this baby and sort out maternity cover again - is that right?
I have just taken 7 months off to have my baby (1 month annual leave and 6 months maternity leave, all fully paid 100%)... I start back at work in 3 weeks - if I go back pregnant (or get pregnant very quickly after starting back), do I just work until I am 36 weeks again and then they have to give me 7 months off again, fully paid?
In terms of your statutory maternity pay, then yes, it all counts as employment so you will be entitled to SMP just the same.
However your employer pays more than SMP so you will need to check your maternity policy and see whether they attach any terms and conditions to the extra pay they give you. I imagine they don't, but you should check anyway. As they are paying over and above the statutory amount they are perfectly free to put more or less any terms and conditions they like on that extra, including having been back at work for x amount of time.