I can only find information that SMP will be extended to a year during 2008, I can't find confirmation that it will be April, as far as I know and can find out, the date for that hasn't yet been decided/approved.
Have found this about more than one employer both after and before qualifying week.
Do you have a limited company for your self-employed work? What is the actual status, how are you paid? Check here for information to decide whether you are truly self-employed or employed by whatever company you actual do work for one day a week.
You may have seen this about atypical workers, but I can't see anything about being both self-employed (if in fact you genuinely are) and employed.
So if it is another employer for which you were also working at the crucial qualifying week, then you can work without it affecting your SMP from the NHS. However, whether it will affect contractual maternity pay over and above SMP is up to them. As I say, I can't find any information saying that the changes to SMP are happening 1st April and I would say almost certainly that if that were the case, there would be plenty of information about that available by now, as obviously that would affect women who are pregnant now. So as it stands, if you are paid the whole length of your pregnancy, that is contractual maternity pay over and above SMP so any conditions on it will be set by your employer.
Whether or not the HR dept is rubbish and contradicts itself or not, you will be able to see the maternity policy that applies to you and check it for yourself. I don't know how you access policies in the NHS, whether there is an intranet available for you to download policies or whether you need to get a physical copy, but you will be able to do that and read it carefully yourself so that, at the very least, any discussions you have with HR are fully informed and not completely reliant on what they tell you.
Hope that helps, I might well keep browsing for more information about being self-employed while receiving maternity pay from an employer, for my own interest and knowledge as much as yours!