Eating weird things is one of the MANY risks with letting cats go outdoors. Not to mention getting hit by cars, attacked by other animals, and picking up deadly diseases (possibly ones that can be brought in to you). Vaccinations don't protect them 100% and you have to be sure you keep up with it every year and flea medicine every month (which can get costly for me anyway).
It's up to you whether or not you want to risk that, I am just telling you because I have mentioned this to other people and they didn't even think of all those factors. If you already know, then nevermind.
Anyways, I am going to guess your cat probably did eat something outside. But call the vet and see what he says. Keep us posted too I am curious if they are ok. I hope they are.
P.S. I know how differently the UK is with the whole indoor/outdoor thing. I am just making some points, please don't take it the wrong way.
As long as they are neutered and have shots, which they do, thats what's important. If you tried to keep them perminently indoors now they would probably freak out about trying to get back out. They could come around eventually if you ever consider it. I would however keep them in the bathroom for a few days until they get better. You don't want to infect other cats. If they are sick, their immune systems are down so they can pick up diseases easier.