2006 TD5 Defender 90
Diff light not working I have checked diff is locking. Looked at dash lights but no individual bulbs all on one circuit board. What is the method of checking the switch at the diff end can it be accessed from underneath or do I need to strip gaiter and move cubby box?

You could unplug the electrical connections and short them out to see if the dash light comes on then. I'm pretty sure that the switch is normally open going closed so shorting the connectors would be the equivalent of diff lock engaged.

Easiest way to check the diff switch would be to remove the gearbox tunnel and test the switch with a multimeter. Put the multimeter on the continuity setting and attach to the terminals on the switch and see if it changes as you engage and disengage the diff lock. Also worth checking the earth wire that goes from the switch to the transfer box casing to make sure it's not something like a bad earth.