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Rat was my first thought but they must be juveniles, with no adults present. My theory is that the reason that they never appear to grow up is perhaps because they are the offspring (there are two), of a local pair of breeding rats, the reason I see no adults might be because they are already busy on the next litter, and the reason they don't appear to "grow-up" might be because once they get to a certain age/size they are seen off by the adults to find new teritory - the next time I see them they in fact the next litter and not the same ones? Intersting colouration though.
Hairless tail and relatively large ears.
OU Certificate in Contemporary Science
Lat/Lng: 50.9, -1.8
OS grid ref: SU1413