Glen - the diff lock switch is on the transfer box not the main gear box, the transfer box is a ref LT230 unit, your main gearbox will be an R380 (crunchy 1st to 2nd gear change!).
If you search for 'LT230 diff lock switch' on ebay you should get a few more results, the part number is PRC2911.
Are you convinced diff lock is working and it is a duff switch/earth connection?