Low coolant level flashing

[croc]

Hi All
On a recent trip on the M5 warning started flashing. Pulled over and waited until things cooled a little and I could check the level visually in the expansion tank.
As there was proper level of coolant and temp normal I continued.
After an overnight stop, the light operated normally but after stopping at the camp checkout it stared flashinbg continuously again. Checked again, all ok.
Drove another 300 miles through the lake district with warning flashing continuously but coolant level and temp normal.
I have removed the level sensor and cleaned the prongs, checked local connections, but cant find relay 43 as in the Wiki, but its still flashing.
After having waterjacket and hose failures in the past I would really like to have this warning system working again! It makes me nervous to not have this operational as it saved the engine several times in the past.
So if anyone can offer a solution it would be much appreciated.
Thanks in advance for any tips. Jon

1985 1.9 DG, later style coolant system with bleed loop around engine hatch. Engine is in good condition with liners and pistons fitted about 5000 miles ago.
There is KSeal in the system but that was added several months ago. 50/50 blue antifreeze now coming to 2 years.

[simony]

[croc]

Thanks for the photo - found it!
I was expecting it to be on the fuse block.
Cheers jon