As Mick (spindle) says, we still have a good (and now increasing) population of reds up here round Aberdeen. By pure blind chance, the greys that were introduced to the city in around 1972 didn't carry the poxvirus. Fortunately also, there is a squirrel-hostile region between us and Tayside, where there are a lot of pox-carrying greys, but they can't get up here without human help. Which unfortunately does get provided by misguided enthusiasts. Even without the virus, the greys had been spreading out from the city and ousting the reds, because they are bigger (typical ***** yanks) and can digest a wider range of foods than the reds. But over the last six years there's been a major project, Saving Scotland's Red Squirrels, which has involved controlling the greys in as many areas as possible, combined with regular surveys of squirrel numbers. I do a transect for this in the woods behind our house and also set up feeder boxes with sticky pads in another woodland to see what species are there. Results have been really encouraging, especially this last year when grey survival over the long, if not snowy, winter was worse than that of the reds.
Reds are now moving back into the city while out in the wilds where we live, numbers have held up or improved; our best sighting is six in the garden at one time.
Back in the early fifties on the Welsh Border, reds were plentiful enough for a local guy to keep half a dozen as pets, but by the late 50s, the greys had completely ousted them.