I was very concerned when the side panel didn't fit, the outlet blocks and handle were in the way. some trial and error found one (and only one) position where you can slot the panel in behind the tap and not foul it. Later I watched a Johnny's Vintage Motorcycles YouTube video that showed me just this trick! Here it is mounted with just a little black gaffa tape on the corner of the panel to protect the paint from accidents.
Saturday I went out and put some more miles on the bike, no dramas but I did get wet. Sunday I walked into the garage to be met with a smell of petrol!
It turns out that there are two types of PTFE tape in this world, and the white one I used, from the roll that has been in my toolbox forever, is what plumbers use to seal against water. That has turned to jelly, and I'm almost certain is the cause of my leak. I have just got back from Toolstation, where I bought a roll of the yellow version, which is for gas and is apparently fuel resistant.