The scaffolding to put the solar panels up hadn't been removed yet, and hadn't quite been properly blocked. So I climbed up to the roof this morning and had a go at sweeping the five inches of snow away that was sitting on the panels, and stopping them from working. In combination with a slight thaw (2 degrees during the day) and doubtless the dark colour of the photocollector helping to warm the surface up, a lot of the snow and ice on the panels was removed. As a consequence, in some weak winter sunshine, we reached a peak power output of 134 W, and managed to harvest about 112 Wh - still hardly impressive, but a step up from the zero that had been generated in the last few days.
Some more technical details about the installation. It uses 10 x 230Wp Schott panels - it is thus a 2.3 kWp (kilowatt peak) system - and a StecaGrid 2010+ inverter. The efficiency of the inverter is pretty high - even at a 5% load, it is nearly 80% efficient; its published efficiency is given as 93.3%, which is the efficiency it achieves at 30% of its nominal power, 2000W.
Rayotec give a written quotation of the expected performance of the system. They estimate (based on a mathematical model) that the irradiation of the PV array over the course of a year will be about 16,400 kWh, and the amount of power generated will be around 10% of this, 1,683 kWh. This equates to about 729 kWh/kWp.