Furball. Always my first thought when a cat stops eating. Out with the furball treatment or tuna in oil. If the problem hasn't gone away, usually in the form of a sausage-shaped fur filled vomit, after a week of regular treatment (the petshop stuff is fine) then consider taking Xander to the vet, till then you may be wasting money on vet bills and stressing your cat with the visit.
Cats do get under the weather for trivial things; they don't always need a vet. You're doing the right thing in posting the problem here, as sometimes it can be hard to tell the difference.
PS try to avoid giving your cats ham, I read somewhere that the preservatives aren't good for cats.
Re-reading the thread, I see that Xander is showing definite signs of furball.