I sent them off for a walk together without me, and things settled down - particularly after they brought out the new squeaky toy they'd brought her. I think part of the problem was that I couldn't help being tense about the whole thing, Tara picked up on that, so was being defensive of 'her' people and 'her' house.
Anyway, they decided they wanted to give it a go, so with some trepidation I loaded up most of her stuff (we'd agreed not to send her crate, she doesn't need it and has been crated enough, and anyway, there wasn't space).
They emailled this evening to say that Tara had settled nicely but that Lola is now being a bit growly and possessive with her toys - not surprised really, she's had an awful lot of car travel today, and now there is a whopping great lump of boxer in her house (Lola is quite a bit smaller than the big squishy girl!)
I do hope my comedy foster dog will be OK once they've had a chance to settle a bit! I *think* she will, both of them are nice well socialised dogs, they just need a bit of time to get to know each other. If it doesn't work out, she'll come back here and we'll try again.
The whole thing was made more difficult by the snow on this occasion, so I couldn't send them off to the woods and it was too cold to chat outside - but I think now that I should not in future have adopters come to my own house to get their dogs. It's too complicated with the cats, it makes it hard to let the foster dog offlead, cos they tend to bolt for 'home' (as Saffy did) if they know the area - and also it's really hard on the poor new owner to have to tow their dog from the house when she is desperately scrabbling to get back there.
Next time I think I will choose a neutral location somewhere that I can either go walking in a different direction, or perhaps lurk in a cafe or pub while they go walking and get to know the dog.