On returning home this evening, I've had to clear up at least ten lots of cat vomit, most of it definitely from today, although some may have been from last night (in the spare bedroom that I don't generally go into).
He's almost certainly thrown up everything he ate this morning, and probably some of the dry cat food I left out (although at a guess Zev has eaten some of that too). Since he's probably been outside, he may well have thrown up out there as well. At least some of it suggests he was trying to go out, because there's a trail all the way to the cat flap!
Luckily, like most cat vomit, it was relatively dry, so hasn't really caused any damage, not that my carpets haven't seen cat vomit before, and luckily they aren't exactly expensive.
He doesn't seem to be exhibiting any signs of being terribly ill, and is moderately active, although clearly not his normal exuberant self. He's mostly just retiring to the heated cat bed and sleeping. He had a slight appetite, but mostly he just ate the jelly bits from the cat food I tried on him, and left most of the bits of fish.
Vomiting once or twice could be a hairball, or something similar, and if he had a full appetite, I'd not worry, but basically he'll probably have had no food stay inside him for 48 hours by tomorrow morning, which is quite a while for a cat, and possibly he could be getting dehydrated (it was hard for me to tell with Talisker, but Kai is covered in too much fur to reliably grab a fold of skin). If he was looking less happy, I think I'd ring up the emergency vets, but his manner seems more or less OK, so I don't think it's desperately urgent yet.
So, I think the cat flap is going to be closed tonight, so he can't wander off and hide anywhere, and he's off to the vets first thing tomorrow morning. I'll ring them up when they open, and take him straight in.