Well that didn't go according to plan.
I spent a chunk of the day continuing the rebuild of the Thorn tandem, then when the rain came on retired indoors to watch the finale of the 5th Tour de France stage. Then I got set up to ride up to Astwood (this includes printing out the timing sheets and signing on sheets). I got half a mile up the road, and found my chosen bike for the evening (my Cervelo P3) was making some loud unaccountable knocking noises. This sounded bad, and didn't appear to be due to the transmission, handlebars or seat, so I returned to the house and grabbed my other TT bike (my Cervelo P5). This entailed some hurried tyre inflation and a change of shoes (different pedal type - old Speedplay X-series). Then a mad dash up to Astwood. Of course, the level crossing was down, to my frustration!
After a brisk ride up to the signing on point (with no mechanical issues up to that point) I rode to the start and at this point there was obviously a problem with the eTap gears. Intermittent, but worsening. Mostly this affected the right hand controls on the aerobar extension and base bar - the base bar switch sometimes didn't operate, and the aerobar switch either (a) didn't operate, (b) did the normal upshift, or as a final indignity, (c) triggered a change on the front mech. The other issue was that the rear mech sporadically shifted to bottom gear.
I came to the conclusion that I couldn't start (or even ride home) and phoned home to to ask for rescue!
As to what's gone wrong, I imagine it must be connected to the wiring modifications I did to pop the blipbox controller into the compartment within the frame. I'll investigate this over the next few days, but I can't quite conceptualise what is going wrong. Front shifting required simultaneous operation of front and rear switches. Maybe water has got in, either from washing the bike of from the rain, causing short circuits?
For the seven riders who started (there was one other DNS), there was a problem on the course - not roadworks, but motor traffic queuing to get into Chicheley Hall, which extended down the drive and out onto the course as far as the junction with the A422. Fortunately everyone got round safely, but times will have been badly affected.