Thank you for your suggestions. I'm sure you are right about the coolant level. I installed a new coolant tank with a warning system some years ago. The corrugated tube in the second photo goes off to the tank but (my age is telling) I cannot remember the connections and can't find the fitting instructions.I attach a slightly better photo which shows the green yellow cable encased in a simple tubing which I am fairly sure I didn't install. I'tried to photo everything before dismantling which took place over a year ago have missed out this wiring. I don't know what the black box in the photo is for even thogh 4 wires connect into it.