Hmmm
im not very technical but something like a small room heater would probably work in a confined space.
In winter times when my band rehearses its pretty isolated and we've used a small electric fan heater in the past soon gets a room cooking.
Obviously the problem of electricity, however you can get portable small gerenators with plug sockets on - my brother has one that he takes fishing to plug a DVD player into. dunno how long itd run a small heater.
Id guess both of these items could be picked up relatively cheap will have a scout around if you want.
would mean you office got pretty warm mark
and you might have to keep the door shut. that or get another smaller 'office' for the paint.
Ojn a completely alternative and more expensive idea, Flametec bags. They are awesome. But would mean having a car down the hill (might not be able to get back up again in this weather)
Apoolgies if this is already known/me talking rubbish