Thanks Pete - unfortunately the fuse box on a 101 is in a very difficult spot up under the dash and hard to get in there. Likewise in that wiring diagram I put up it would seem that the gauge and sender and the fuel pump are on the same line and I have power to the pump but not the gauge - very confusing.
But you were are correct - bit the bullet and pulled each of the fuses and yep - as you described the filament in one was intact but the end caps were not. Putting in a new fuse and I now have the difflock light back but not the fuel gauge - very confusing.
So back to the wiring diagram - searching my documents I came across another wiring diagram that is much clearer. That smudge between the fuel sender and the fuel pump is not a continuous line but earths

. So that clears that aspect.
I can now track the power from the fuse to the gauge behind the dash and then to the sender - might try that tomorrow. However I did pull the wires off the sender without taking a pic first so may have put them back incorrectly - so will look at that first - I have a spare sender so check it all with that.
Thanks for you input - prompted me to have a look elsewhere. Much appreciated.
Garry
PS - Mick - thanks for your comment - yes you are also correct
