Maternity cover: company has created a new role and will be paying someone a higher salary

I'll try and make this short.

My partner will be going on maternity leave in a couple of months, however, her employer of 5 years are not intending to find a maternity cover for her. Instead they plan on hiring a new member of staff as a manager who will have their own responsibilities as well as having to do the work my partner would normally do. My partner is not currently a manager, so the member of staff which her company hire will be paid quite a considerable amount more than her salary.

The only maternity pay her company are offering is statutory.

Is this legal?

Thanks in advance. :)