Sadly, I didn't get a photo of the funniest bit, which was when I moved the sofa out slightly in order to see if I could see the mouse and Cassie started to squeeze herself into the gap. I wish I had a picture of her front half stuffed into this small space and her back half spilling out because it was absolutely hilarious.
I let the cats stay while we cornered it as their infalible instinct for where it had gone to ground again was invaluable. Cassie did manage to grab it at one point (doing it no harm) but she dropped it and then lost interest, presumably feeling her work was done. Finally we managed to force it into tupperware and I dropped it into the neighbours garden, relying on the fact that it hadn't been naturally occuring in our house anyway but brought in by Cassie so hopefully it's unlikely to make it's way back inside. And even if it does, Cassie will waste no time pointing it out.
Oh, also, our opinion that Cassie is responsible for all the mice was reinforced by the fact that the mouse ran away from Cassie and circled round Charlie's paws while Charlie looked baffled and wondered where it had gone before moving to under the side table. Right under her paws, I swear. She'll never make a mouser :)