Our garage has the central heating boiler in it so that keps the garage in a nice damp free enviroment, but if I have to work in there during the winter months the tumble drier goes on for a few mins just to bring the temp up to a slightly more comfortable working temp
Homebase and b&q do excellent flat-panel oil filled heaters which you can easily wall mount or leave free standing. throw off more than enough heat and some have thermostats although a simple plug-socket-type timer switch is all i use. keeps the condensation away (on low setting) which is the most important thing. cheap to run too - steer clear of electric fan heaters or gas.
That and/or a huge glass of whiskey will keep you nice and toasty although you may get forget what you were doing in the first place after a few too many of these.
Might be a good idea to insulate the garage before you go wasting a lot of heat ,get something on the floor ,that wil prob make the biggest difference ,stop any draughts as well , then get a heater
Just remember that a intergrated garage is far easier to heat than a detached garage. The heat lost on some garages is that great, it bearly raises the overall temp, just the local temp around the heating source.
My garage is a single brick jobbie, crap for heat retension. I use to have a big problem with condensation. This was always due to the bike beeing very cold, hovering around the 2-3 degree above freezing and the suddenly the ambient temperature rising. The warm air started to condensate on all of the metal parts of the bike. I cured this by building my own bubble for the bike. I had some ISO super9 kickiing around, whick I believe is the same stuff used to insulate the shuttle craft. Its mainly 25 layers of foil and 0.5mm foam. I just wrap the bike up in this till the spring.