All is well.;D Connected up the selector valve, started the engine (no leaks, hoorah) and got a good continuity reading through the pressure switch. The hoses that should've got hot, got hot; the others stayed cold. Switched off and tested the pump. Again, good pressure continuity and the other hoses that now should've got hot, got hot whilst the first ones cooled. Wired in the pressure switch to the 6-pole socket, connected up to the RV, took a deep breath, and..... the controller liked it. 8)
I fitted the inline magnetic filter between the transmission and the selector valve which may do some good to the tranny generally. I'm obviously happy to have it all working again and to have learnt some good things along the way. I like the way it makes you less frightened to deal with stuff too. So as ever, big thanks to everyone for jumping in with the help!
Afterthought
As I lay awake at 5:00am this morning trying to ignore the thunderstorm, it occurred to me that after having towed with the engine idling, it would be useful to use the pressure warning system under those circumstances without the pump running. Simple solution - fitted a switch in the engine bay to stop power to the pump (e.g. if it goes faulty again) but leave the power to the pressure switch. Result is the monitor in the RV does its job with the car engine running so no more worries about it stopping without warning.
Dougie.