I am 4 months into my maternity leave and had intended to return to work in May if my employer agreed to me working part time (3 days per week 9-5). The person covering my position is currently working 3 days per week. My application for part-time hours has been declined as the new department head has informed me in a meeting I requested that she intends to change my role considerably from one of admin to re-training me to deliver training courses on areas previously covered by another department and onces in which I have no current expertise. Even though I have extensive training experience this is not the role I was performing pre maternity leave. My employer has offered me a 5 day week 9-4. Do I have to accept these new working conditions and go back in October full time, or can I challenge their decision based on the changes they wish to make and that my temporary replacement has been doing my job part time?
It is unclear what hours you worked and how many days a week you worked prior to going on maternity leave, but I am assuming that you were previously working full time i.e. 5 days a week 9 to 5. The significance of this is that you have the right to return to the same position (same hours, days etc) as when you left. The fact that the person covering for your position is currently working 3 days a week is helpful to your argument that you should be permitted to work part time (3 days), but it does not give you a right to do so.
Leaving aside the full time/part time question for a moment, in relation to the change of role, they should be offering you the same role or, if that is not available a similar role. It appears that this is not what they are doing and they are trying to change your role without your consent. I suggest that you can challenge the decision on their part to change your role which appears to be unilateral on the following bases :
(1) The role that they are proposing is not the same as the role you left. It is not even similar. You should give the reasons why you do not think it is similar.
(2) You have a right to return to the same or similar role following maternity leave.
(3) Even if they did argue that it was a similar role and that the business requires them to reorganize the role in this way, they should still be consulting you on the changes and trying to accommodate any suggestions which you may have.
(4) Regarding the flexible working, whilst you accept that it is not your right to work flexibly, it would be discriminatory and also contrary to the Flexible Working Regulations if they decided to turn down your request for flexible working without proper consideration. They should bear in mind that any arguments about 3 days not working will be called into question because the stand in is 3 days a week. If they argue that the stand in is simply working 3 days as a stop gap and the real role is 5 days, you should say that you feel you are capable of making it work and that they should give you a six month trial period.
You could choose to raise these points informally or you could put it in writing requesting a meeting and either express the letter as being a formal grievance or choose not to initially if you do not want to cause too much controversy.
I am not sure whether you really want to resign over this issue and make appropriate claims. Should you choose to do this then you would need to not only allege that their conduct is discriminatory, but also that there has been a break down in trust and confidence. This is quite an extreme measure in this economic climate having just got back from maternity leave as well, and I could not recommend this. Your better option is try to negotiate a favourable position coming back and if that fails then negotiate a settlement rather than fighting it in tribunal.