The ongoing cold snap shows no sign of abating, not at least for the next week anyway, so I decided that something ought to be done to improve the rate of heat leakage from the 'shop. To that end, the draughty opening window has been completely sealed with silicone...sounds a bit drastic, but it's due to be taken out soon and replaced with a double-glazed insert.
The door has also been sealed up with some 'P' section rubber sealant strip, so it now closes very tightly and there are no gaps anywhere. The main form of heating is a 1.5Kw oil-filled radiator set against one wall and this has been switched on using a plug-thermostat, which has been positioned some 2m away on an adjacent wall, with the dial on '1' which ought to correspond to roughly 10deg C.
I switched it on around three hours ago and noted a three degree rise just now so I'm hopeful that when I check the temp tomorrow morning, the magic figure will have been reached and as the 'shop is well insulated anyway there shouldn't be too much trouble in maintaining it.