Thanks so much to everyone who sent good vibes, offers of help and great suggestions, it really does make you realise we are creating a real community here in Blogland.
Heard from mum today that dad came out of hospital. He's still on antibiotics but the doctors presumably think he is well enough to come 'home', though he has strict instructions to REST. Mum said he's in good spirits and seemed quite well today. She has also managed to get a better room at their hostel, ground floor (yay, no stairs!) and with an ensuite. So they're going to hole up there for at least the next two days, then if Dad is OK, they'll pick the hire car up as originally planned (taking a taxi to the train station this time...no more Tube!), and then head off south west and find a nice B&B and stay for a week or so (I hope it's more, but we'll see). She says the weather is currently really nice in London so that's also a positive, they might be able to get out and go for some GENTLE strolls in the sunshine.
So that sounds a little better than it did yesterday! But I'll feel much better when I know they're at a nice B&B somewhere in Devon or Cornwall (most likely), putting their feet up and relaxing and not going anywhere much at all!