I went into the garage last night for a play (finish fitting the Heilbars to my Breva 1100 that is) and froze my proverbials off in pretty short order!
So should I go gas fired - https://www.machinemart.co.uk/p/clarke-little-devil-2-propane-fired-space-heat/
Or all electric - https://www.machinemart.co.uk/p/honeywell-heavy-duty-utility-heater/
Garage is about 20’ x 10’
Cheers
Yes fan heater, blows the hot air around, heats up fastest. Used them for years.
Ideally get it going for an hour before you want to start working in there.
Also great for the summer (heat off).
Definitely electric.
Gas produces loads of moisture. This is a worrying read on the topic http://www.swale.gov.uk/condensation-mould-heating/
I’ve got one of these and it works really well https://www.amazon.co.uk/6800BTU-Industrial-Workshop-Heater-Electric/dp/B00J25O9WO/ref=sr_1_3?ie=UTF8&qid=1485870553&sr=8-3&keywords=fan+heater+workshop
Good luck
Steve
It was the moisture bit that got me thinking
I’ve got a small fan heater and the noise it makes is a bit irritating.
Do the bigger ones run a slower - higher volume fan so theyr quieter?
Cheers
It’s not silent but not too bad. Doesn’t drown out The Archers 
I put an old bottle jack on the top to reduce the noise. An electric fan in a metal box 
The jack works well.
My mechanic has the same model and has mounted four round door stop rubbers on the bottom; he won’t agree that mines better!
All the best
Steve
Plus electric doesn’t risk carbon monoxide or burning up the oxygen in an enclosed space.
Have a 2KW electric fan heater bought for £19.99 on E-Bay, heats the garage in about 20 minutes and can then be turned off, also works as a fan to blow the exhaust fumes out when running the bike indoors.
Am a fan of portable oil radiators…on all the time thermostatically controlled they keep the place comfy and are not ridiculously exoensive to run…
Being a belt and braces sort of bloke, a half an hour before I go out I switch on both the fan heater and an oil filled radiator. That makes the place habitable. The fan heater can then go off and the oil filled radiator just keeps it topped up.
I used to have one of those portable calor gas (bomb in a box) heaters. It warmed the place a treat but the condensation was terrible. Also I like to clean things in petrol.
As has been said, small electric fan heater is the way to go. I’ve had one for years, free to run as well, if like me, you have a streetlamp that’s hard up against your property…
Al.
+1 Mine works a treat.
In addition to a £10 fan heater from Currys, I fitted a rubber sealing strip along the bottom of the up-and-over door, and lined the door itself with aluminium insulation… It’s nice and warm, but hard work lifting the door up to get the bike out!
When i used to work in an icy cold workshop, I fitted a tubular greenhouse heater under the front of the bench down near my feet, with a metal cover for safety and a frost stat it kept the ice away and the gentle flow of heat kept my feet warm and wafted up to my hands
they are 60W per foot so cheap to run
I also fitted one inside a classic car to prevent condensation and ice damaging the upholstery