I'd be extremely surprised if you've seen a mole in your garden. There are people who've had molehills all over their gardens for decades but they've never actually seen a mole ;-)
Moles are completely harmless to people (except those people who're driven to distraction by molehills in their lawn!) and pets. They're only about 15cm (6") long and live mainly on worms. They're almost blind and rarely come above ground. A cat might easily kill a mole but a mole can't harm a cat.
You can learn about moles here:
http://www.wildkids.org.uk/woodland/mammals7.h tm
If you've got a mole in your garden, you'll know because of the mole hills. The Royal Horticultural Society have advice on dealing with the problem:
http://www.rhs.org.uk/advice/profiles1000/mole s.asp
It's far more likely that what you saw was a large rat. If so, your cats will be delighted. (The best way to get rid of rats is to get a cat). The only slight risk to cats is that, if someone puts poison down for the rats, the cats could eat the bait or (more likely) eat the dead rat containing the poison.
There are no native mammals, other than foxes, which pose any real risk to cats. However, if you live near to water, you might have seen a mink. (These are now found throughout the country, after their ancestors either escaped from, or were released from mink farms). I've never heard of one attacking a cat but it might occur. Information about mink, together with advice on dealing with them, is on the Defra website:
http://www.defra.gov.uk/rds/publications/techn ical/tan_02.pdf
Chris