Following on from the last log and the observation that, even with cooling, the coldest end of the temperature tower still looked ok (180C), I checked the nozzle temperature with a thermocouple and found that it's consistently 7 to 8C hotter than the machine indicates. So I re-jigged the temp tower G-code to account for this, and also changed the range to 170-210. Here are the results (with external fan cooling):
All settings give essentially the same results. What isn't terribly clear in the photo is the slight stringing in the bottom 3-4 sections (210, 205, 200 and 195C). Also the side of the tower that faced the fan has much better bridging than the 'down-wind' side. Just goes to show nozzle temp means nothing - cooling is everything!