they're not going to be too pleased
But if it's the law then they'll have to suck it up.
Similar thing happened to a friend of mine. She went back to work after her first child and 6 months maternity leave, and 6 months unpaid maternity leave, and accrued leave (almost 57 weeks in total since she had gone off for maternity leave): within 4 weeks she found out that she was pregnant again! She took maternity leave immediately (even though she was not in an aviation industry) because she knew that her department were going to be made redundant en masse before she would have returned from maternity leave. It made sense to me, plus she got her full redundancy package!
Check with you union rep, and with HR.
I think we are more governed by EU law than UK law these days.