Post Script
I was sure I had seen something recently about substitute sender units and have just stumbled across the article in Nov/Dec FP (p11) by Ross Barratt.
He confirms that a replacement is available and that the correct resistance is 0-84 ohms, so my 1-86 ohms is close enough.
Incidentally, my sender was not working when I got it but some careful cleaning of the wound resistor and the wiping contact restored it to working condition.
Roger Grix