It turns out to have been 83.7 miles according to the GPS - but I always think it under-reports slightly (or maybe our bike computers both over-report...)
The rain on the way into London was as bad as I can remember cycling in - all the drains in North-West London were overloaded, with gushers coming out of manhole covers, and streams that would have sustained sticklebacks flowing down several hills. Thanks to @
CharlieB for his suggestion of a detour through the Moor Park estate to avoid Batchworth Hill - it's a good route (although it would have been a good idea to change the batteries in the GPS before we left home rather than waiting until we were enjoying a shower outside a mansion on a private estate with neither streetlamps nor pavements).
I've never enjoyed a soggy BLT as much as the one I wolfed down at Marylebone, and the rolls at Stock were absolutely perfect. The breakfast at the Rose is pretty good, too.
Amazingly I was only cold for about 5 minutes after leaving Stock. I'd done the first half of the ride in long-sleeved base layer, long-sleeved (but thin) top and lightweight showerproof running top (which kept me pretty dry - only my arms were soggy) over shorts. Adding leg- and arm-warmers, replacing fingerless mitts with waterproof gloves, and putting dry woollen socks on in place of soggy "waterproof" jobs seemed to do the job. Perhaps I'm just lucky - or still well-enough padded that I've got enough of my own natural insulation.
As @
rvw has said, we had no problems at all with the bike on the trains home. I think that Southend route is openly tandem-friendly, while the chap on the manual gate at Aldgate didn't blink an eyelid as I wheeled my long vehicle through, and the driver looked impressed when we told him what we'd done. The new Met line trains on the underground (soon to arrive on the Circle, District and H&C) make life very easy for the cyclist - there's a bank of three tip-up seats in every carriage for luggage and pushchairs which are perfect for bikes, and a tandem only blocks off one more seat. To avoid a final lug over the footbridge at Amersham we got off a stop early, for level access to the road.
Apart from hanging around in the chilling air in Chadwell Heath that was a lovely night out. I need to adjust the light on the tandem a bit, and the back brake cable is in dire need of some lubrication - but the Thorn is a perfect bike for wet-weather riding.