Well, the good news is Lucy's completely back to normal, making me think it wasn't a stroke at all but the last of this virus knocking her off balance.
She's bouncing around again, back to her third kittenhood, playing pounce on my feet under the duvet, rolling over and over so I can tickle her tummy - then she can kick me with her back feet, jumping on and off the bed without falling over.
I've also noticed that she's breathing 100% through her nose again which a few days ago she wasn't doing - still occasionally breathing through her mouth so it looks like her sinuses have cleared up.
I decided against taking her to the vet today as it is very cold here and I felt that taking her to the vet would be very traumatic for her and could perhaps do more damage than good at this stage. We're all keeping a very close eye on her and at the first sign of trouble she'll be off to the vet again but at the moment she's acting like a cat half her age so she's obviously feeling much better in herself. Long may it continue.