Might be worthwhile asking Kirst for advice. I think she deals with this sort of thing at work.
Indeed, I am an OT.
Sorry Sam, I missed this thread altogether.
Your best bet is to contact your local social work department and ask for an OT assessment, but they probably have a long waiting list, and bathing usually isn't a priority. You could ring the district nurses at your GP practice - sometimes they can access simple bath equipment. If you don't want to wait, you might want to think about private purchase.
Which bit is difficult for you? Is it the climbing over the bath to get in, standing in the bath for a length of time to have a shower, or sitting down into the bath to have a bath, or getting up to standing when you've been sitting in the bath? Or all of that?
As long as your bath has a decent lip on both sides, you might manage with a board or a board and seat.
Something like this fits across the bath (they have brackets underneath and you secure them so they don't scoot all over the bath) and you sit on it and swing your legs over the side. Then you can either stand and shower, or sit and shower, then to get out again you sit back on it and swing your legs back over. They come in different sizes for different widths of bath, and they're about £20.
If you prefer to have a bath, you might want a board as above, and a seat. You would use the board as above, then lower yourself onto the seat, have your bath, then raise yourself back onto the board and get out again as above. Seats are about £20 as well.
Boards and seats work well for some people, but you don't get a proper soak in the bath, and as you're having multiple muscle problems, you might not manage them when you're having a bad day - you might be better with just the board and forget having a bath. If you've got loadsamoney you might consider a powered bath lift - they take you right to the bottom of the bath and some have reclining backs so you can get a proper soak. They're about £500 though. You have to be careful that the flaps don't get stuck in the bath handles as it comes back up though, or you're facing some expensive repairs.
Oh, and there are swivel bathers too. Again, you need a decent lip on both sides of the bath, and they're about £200. You fit it to the bath, sit in it, swivel it round over the bath (lifting your legs over) and then have your shower above the bath and then swivel back to get off.
If you have a Disabled Living Centre nearby (the social work department will be able to tell you) you can go and try things out before committing yourself to buying something. Independent advice from a DLC is much better than "advice" from a sales rep. Let me know if I can help in any way.