I was half hoping nobody would turn up for this ride given the amount of rain in the previous 48 hours and the fact that it was still drizzling a little. But there was Ken eager to conquer Dragon Hill and Andy arrived shortly after. We set off on the modified route, modified to avoid the A417 closure at Challow Station. The drizzle stopped and there was even a little brightness as we made rapid progress through Childrey, Sparsholt and Westcot. Ken sailed up Dragon Hill where we encountered Larry waiting for us. A cautious descent followed and we swept into Uffington where my rear tubeless tyre started spraying white sealant liberally over the road and the underside of my mudguard. It quickly became evident that the Caffelatex sealant was not up to sealing this puncture, so an injection of Zot catalyst was attempted. I suspected that insufficient sealant remained in the tyre at this point for effective repair so I added more sealant through the valve (thanks to the Gentleman outside whose house we had stopped, who provided both track pump and valve core remover). The tyre now held up to about 30 psi so we thought we would set off back to Wantage. It did hold until Baulking where some pumps from the hand pump got us a further km. At this point, with the light fading, I called for the fourth emergency service (Pam) and the rest of the group made their way back to Wantage. By good luck I didn't attempt fitting of an inner tube which would have been unsuccessful (see below), and by this stage there was possibly insufficient sealant for a repair using an anchovy.
As I needed the bike for a long ride the following day I replaced the tyre that evening. This was straightforward except for the unseating of the old tyre from the rim, onto which it was very firmly glued - use of a G clamp necessary! Attached photo shows the c 5 mm cut. My confidence in tubeless road tyres is now severely dented.
Well done to Ken for proving to himself what we all knew he could do, and thank you for your patience and Andy's advice on matters tubeless.