Totally forgot the floor!

It's a 6" slab, so
Area = 36.75sq/m
So allowing for U value of 2, and assuming ground temp of 10deg (I've knocked a couple degrees of for being further north than you!)
Heat loss = 36.75 x 2 x 10 = 735W

So total heat loss through conduction is 2295W


The other building is unheated and uninsulated, and it's a single brick construction with corrugated fibre roof, with two set of badly fitting double doors, so is as good as being open to the elements!
As for the space heater, it's somewhere in the 15kW range. Get's the air upto temperature quickly, but smokes the place out andgiven the amount of cold metal in the shed, you're frozen again within minutes of turning it of.

I know drafts are a major issue, but it was interesting to see just how little of the heat is actually lost through the surfaces, so number one priority is getting a door fitted between the two buildings, followed by some draught free doors to the outside. Then I'll work on getting the walls lined and insulated, as the plan from day one was to fix 3x2's to the external walls and have insulation sandwiched behind some 1/2" or 3/4" ply.