As a general rule of thumb, to heat a space to 18~20 degrees C you'll need 35~40Watts of heat per cubic metre.
It's a very very rough guide but works well. Obviously, if your talking about less heat down to say 10C, you'll probably need approx 20W/M cube.
Best to "lessen" the space and put a bubble around the bike with suitable vent at top.
Tubular electric heaters output about 20W/foot and in answer to the next question, a 6 foot tubular heater would cost approx 1p per hour to run (6ft * 0.02kW * £0.08p/hr).
Best just keep it in the kitchen and put the wife in the garage.
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