This update should be beginning with a smug account of how the 190E now fires on the key in sub-zero temperatures, but as much as I'd love that to be the case, it isn't quite there. Indeed, there's more work to be done in this department.
That's not to say there hasn't been some progress. I sourced a new coolant temperature sensor and swapped this myself - a very simple job - and the situation seems to improved slightly. Instead of multiple turns of the key before catching, it now (usually) catches on the first turn, but settles to a very lumpy idle before quickly warming through.
So, it suggests the problem might be more than just the sensor, and that's exactly what I said to mechanic Jamie at Simply Automotive in Poole as he heaved open the 190E's bonnet, looked through outstretched arms at its puny 1.8 and sucked his teeth. Clearly, I'd driven it to him, so a cold start wasn't possible, but we did dismantle the airbox and get to the fifth injector, which was working just fine.
After various circuit checks and with little else we could do there and then, I booked the car in to be left with him for a longer period in a couple of months and jumped back on Google. Another possible cause seemed to be the car's over voltage protection (OVP) relay, its 10 amp fuse often blowing and not providing enough oomph when it's most needed. Of course, there are also question marks surrounding the fuel lines, pump and filter, all of which will be assessed soon.