sex discrimination question

[ohdearwhathappened]

name changing regular

I lost my job on maternity leave.

There may be specific differences in my terms and conditions to the usual but I gather these can't replace statutary (?spelling)

My union says that it can't be sex discimination because a (childless) woman got my job, I worked part-time the job was changed to full-time (all my employment was full-time prior to that job) my boss asked my prior to the interview whether I wanted to work part-time

the bloke covering me also applied and didn't get it they say the fact that he worked part-time (although he said he would only work full-time) also counts against me

so my question is if a woman takes the job of someone on mat leave does that mean it's not sex discrimination?

thanks