Defender TD5 not Cranking

I have a 2004 TD5 110.
She has always started on the button until recently.
Flat battery etc so after a few days on a trickle charger she was fine again.

Having had a gearbox swap I needed to setup the diff lock linkage and install a new switch.
Then needed to start her up to move her and disengage the diff lock switch (I read that the wheels need to rotate at least a quarter of a turn to get the light to go out.)

This is when the starting problems began.
All electrics work but the engine will not crank at all. Fan, wipers, lights & everything else works ok.
The starter did not turn at all.

Having read that I should change the battery in the key fob as it might be triggering central locking to open but not disarm the factory fitted immobiliser, I changed the battery in the fob and she cranked up and ran like a dream.
Til the next attempt to start, nothing again.

Has anyone any ideas?

Check the earth straps are secure. Gearbox and engine. As gearbox has been worked on that strap may not be connected properly, Pull starter solenoid in fuse box, (No 2) clean contacts and replace,

Also check the fuel solenoid.