Yes, with a multi meter. I disconnected the wires at the sender on the tank. When I measured the resistance across the sender terminals I got about 9 Ohms. The tank is brimming and the senders I have seem to range about 20 Ohms full and about 300 Ohms empty. Actually this sender is the lowest resistance at full of any I have come across. I was worried it may be drawing too much for the regulator.
Then I checked that the wire in the back of my gauge was the same one a connected to the sender as one was Green-Black and the other Black-Green

. I did this with a long peice of wire and checking continuity on the meter. Beep meant the harness was made by the devil but still working.
Then I measured the voltage at the regulator. There are B and I markings. With the body of the regulator earthed and ignition on, one should read battery voltage and the other should read 10 Volts. Mine read zero.
I grabbed another off a spare speedo and swapped the wires over then tried the new one. My digital meter flickered as the regulator did its thing but it was near enough to 10V (DC setting on meter).
Put it all back together and the needle came up like it had just taken a blue pill.
I think I may have blown up the regulator when driving with the speedo hanging out of the binnacle as I tried to diagnose the lack of speedo worky. Maybe the fuel gauge terminals shorted to earth in the dash rail?
M
Thanks for the tips.
I have placed an order for a new one.