I'm an idiot. After a slightly hair raising moment on the ride home today in freezing conditions and sleet I decided that maybe the 700x25 slicks weren't the ideal tyre for the winter. So I get home and start putting on a set of Conti Speed Kings I have in the shed, nice fat (35) tyres and quite soft rubber. Turns out they're a mm or two too big for the mudguards, so now I've put the original tyres back on and I think I'll leave it as I spent this months bike budget on new brakes and pair of long tights.
If it snows I'll just take the mountain bike and ignore the 30 minutes I wasted changing tyres around.