If I'm being honest, I don't like it, but they appear to be playing a blinder. They have given business reasons for the refusal. No employment tribunal will dispute an employers business reasons - nobody but an employer can judge their own business reasons.
My guess is that they don't want you back and don't want to pay redundancy pay. And if you can't overturn the decision with the employer, are you in a position to return to call their bluff- because I think they are betting on you not being able to do that.
Because I can tell you now what their defence to any claim will be - "hi judges, but you can't tell us how to run our business so what we say we need is what we need; but then poor us are struggling financially so we couldn't afford to replace Hope1987 even though we desperately needed to, but look at all the people we had to make redundant so you can see why there was no maternity cover and we were just SOOOO happy when she was able to come back..."
You can't call their bluff and you walk away with nothing. You call their bluff and probably can't sustain it, you walk away with nothing. You go to tribunal, you'll probably lose, so worst case scenario is they pay you LESS than the redundancy payment to go away because that's probably the best offer you'll get.
I hope you are are in a union. That's no guarantee of anything because the law probably isn't on your side here, but if there's a way through, it'll take union resources to find it; or to scare the employer enough to back off.
Sorry. Wish I could be more optimistic, but I don't see this ending well UNLESS they accept your appeal - which won't be on legal grounds, but on the basis that you get a more sympathetic hearing.