Pertinax I have not seen any for ages!! .... When we were at London Zoo in the summer in the Clore one of the volunteers pointed out a rat that was as big as a small dog! whilst commenting on them having all of London wildlife in the zoo but had not seen one that big before!
I was close to pronouncing Colchester a rat-free zone.
I don't remember seeing any at ZSL but have watched them by the bridge over the waterfowl enclosure adjoining the Lake in the Park. Here they feed on the bread leftovers thrown to the birds. I guess the Regents Park Canal is prime habitat also.