Ok, unplugged the black temp sensor and bridged. As the gauge was already at max nothing happened, but when I removed the bridge the gauge steadily dropped to min and light went off. When I put the plug back on the light came back on a gauge shot to the top.
I bridged the coolant bottle sensor when the light was off and gauge was at min. And it did make the light come on and then off again. So, I now believe, with your help

, that the black sensor is knackered. Is that right?
Local stealer has one in for £24 or GSF for £8....leaning toward stealer.
Many thanks for help.....I need to post a pic of car once I have done everything!
Just need some genuine mats, find a new tape holder for dash and then a nice powerful cassette radio with green backlighting that will fit!
Rob