It is indeed a 12 volt LED, so hopefully none of that confusing stuff about resistors applies 
You lost me on the second sentence, Group4_Mark2 
I just chose blue cos’ it looks cold, not for any technical reasons 
Had to go and get some more yellow crimp connectors at lunchtime, so got a multi-pack of lots of flavours and a packet of yellow piggy-back ones. This meant I could have a convenient take-off for the LED from the fan wiring. The wiring for the LED’s not going to stay this short, it’s just for testing. 
When installed, it’ll be a wire from the positive on the fan that goes through to the LED mounted on the dash, and from there off to a convenient earth point.
Following yesterday’s methodical regime, I brewed up again 
, refilled the “radiator”, the thermostat clicked, the light came on and the fan went whizzy, whizzy, whizz 
I then topped up the “radiator” with some cold water, the temp dropped, the thermostat clicked, the light went off and the fan wound down 
Today’s schematic then looks like this…
Thanks again for everyone’s help and encouragement on this – TS is a marvellous place