Yes I considered a de-humidifier but A) it will take up some space i'd
prefer to use for something else (garage is quite small and full!) B)
They are expensive(ish) to buy and C) they use a moderate amount of
electricity.

I was thinking if the garage were very well vented then I probably
wouldn't have the condensation problems and since the condensation
problems don't occur while I'm working in there I could seal them and
heat it while I'm in there and then open them when I leave which
should sort out the problem with the minimum cost and space.

There is a window in there but I dont want to open it as it's a bit of
an invite to burglars