My last cat was ancient when I adopted him and he had a smelly breath. The vet scaled his teeth and all was well.
However the next year the smell started again and the vet checked his teeth and said that they were fine. His gums, however, were slightly inflamed and the vet said that cats can get chlamydia and herpes infections in their gums. I knew that Koala bears got it but just assumed that it was a STD.
Tiggy is still a relative youngster but it is worth bearing in mind. Could be a tummy problem too.
I hate going to the vet - it is about 100 yards up the road and I walk, but cats seem to get really heavy when you put them in their carriers. I am looking for one with wheels - or a pushchair. ;-)