I think your question is 'do I have a right to return to work on a part-time basis after maternity leave'. The answer to that is 'no' but your employer must seriously consider such a request and give reasons why not if refused.
This wording : You have the right to ask for part-time or flexible hours and your employer has a duty to seriously consider your request. Your employer must have a good business reason for refusing. You may be able to rely on sex discrimination law if your request is refused and you do not think your employer has a good business reason for the refusal. Since April 2003 the right to request flexible work involves a clear procedure, which both you and your employer must follow. You should make a written application and your employer must then arrange a meeting with you to discuss it within 28 days, unless your request is agreed at the outset. .............. came from this website :
http://www.askbaby.com/returning-to-work.htm
........ and it summarises neatly my understanding of it. However the NHS MAY have specific policies about this (more flexible), and one of the NHS employee on here may have more knowledge about the NHS.
I think you would be best asking for part-time work - not a straight change to 5 morning shifts only. You have no rights to demand the employer succumbs to that.