Thanks for all the suggestions, guys.
To clarify: I'm vegetarian, so I don't mind the pesto having cheese in. However, if the cheese is made using animal bi-products enzymes [instead of vegetarian alternatives], then I don't eat it.
Unfortunately, most of the kinds of cheese they use to make pesto isn't specifically vegetarian, which means they'll probably have used enzymes from an animal source to make the cheese. This covers every pesto I've found in Tesco, Aldi and Morrisons, except some of the tomato/olive pestos.
Ooh, I must find me a Sainsbury's....