How much of this is my responsibility?

[Backbag]

My parents. I just leave them to get on with things. They're adults and until recently have been in good health, more likely that I'd go to them for help than the other way around.

Recently Dad has had some health issues that are affecting his mobility, leaving him very dependent on Mum. He is not a good patient! She has aged overnight.

He won't consider any adjustments that would help him, and therefore Mum, e.g. mobility scooter or adaptations to the house for bathing etc . Mum wont consider anything that isn't what he wants. I can't "make" them, but it does feel like "someone" needs to put their foot down....

This sounds very difficult. I guess the best thing to do is stay engaged and try to steer things in the ways that work best for practicality. But ultimately those decisions are theirs to work out. This might be frustrating at times.

None of it is your responsibility in law I don't think. It sounds like your relationship with your parents is shifting and you're particularly worried about your mother and how she's coping. It's a horrid time in our lives when we become parents/carers to our parents! For all parties, especially when someone still has all their mental faculties.
I'm guessing that noone can do anything while they both have capacity to make their own decisions. Even if those decisions aren't particularly helpful or are poor ones. If you think one or both of your parents is at risk because their decision-making is poor, look into policy and process around vulnerable adults. Something there might be able to swing into place, although probably if your parents agreed to it.