I learned from the folk on the AOMCC forum that the mixture can be adjusted by changing the float level, as in other carburetters. I hadn't thought of this approach with the Solex because the float acts directly on the valve and there is no arm to bend, for example.
However, the collective consciousness revealed that the float valves could befitted with several washers if necessary, to lower the valve and hence the fuel level, leaning out the mixture. Like many others, I had replaced the fuel valve fibre washer when I rebuilt the carb, with a single washer of about 1 mm thickness, but apparently there can be up to 3 mm of thickness there.
Since my bike was using quite a lot of fuel and, judging by the state of the silencers was clearly running rich, I added a second washer.
A plug chop revealed that the change had been a success. Previously, I had been running NGK B5HS plugs as my B6HS, equivalent to standard fitment, had been sooty. After the washer was added, I was able to revert to the B6HS and they were a much better colour.