Thanks Alan and Peter.
Early last year, I made up an angled piece of plumbing copper [28mm] as a simple
connector to the thermostat. It was easy to solder a T into that.
I made up another 15mm T at the heater pipe joint [which was already there,
conveniently], and linked them together.
I ran the engine from cold, and the warm-up time is much reduced. I'm getting heat to #4 almost
instantly, whereas before that pipe had zero flow.
As it happens, the new rad switch [88/83] arrived, and fitted, now the fan is kicking in much earlier.
I appreciate that this mod won't work for most, but it's just one more way to do it.