Thank you everyone for your replies - I've only just seen them all. Have been busy chasing up other agents and viewing a couple of flats. I've just stopped mentioning the wheelchair and I haven't faced any problems as a result (!). The flats I've seen so far aren't quite right for dad and his wheelchair (really awkward shaped rooms/corridors etc) but I have another viewing tomorrow so fingers crossed!
I think sheltered extra care housing would be ideal if they qualify (the ones I've seen so far need a link to the local area etc - but tbh I haven't done a great deal of research) but I know mum would be too... proud?... to consider that at the moment. I'm not sure if pride's the right word but she's just so resistant to the idea that they might need help. I'm hoping just to get them here, give mum a break, share the responsibility and everything, and then she's a little more settled we can start planning for the future together. Obviously that all depends on how dad is but because he's early onset things seem to be a very slow progression.
(Weirdly I was also at the post office earlier today and a man on a mobility scooter got stuck going into the post office and asked me to give him a push, which I was very happy to. I've never noticed until recently how inaccessible (or at least hard to access) so many places are and now I'm seeing evidence of it everywhere!)