Colin,
Something else, on the dashboard in the centre is a socket for a 2-pin plug to use an inspection lamp, these 2-pin plugs and sockets were a common household item in my younger days, for table lamps, etc. Finding plugs and sockets was not so easy, but I did find one at home then some popped up on Ebay. I recall there is another socket on hull offside under the armour above engine bay, there was another in the turret which fed the spotlight, so the crew could unplug and use a inspection lamp if needed. The spotlamp is a commonly used Simms and they still appear at swapmeets and dealers at times. also fitted to Matilda.
Here is a picture of the 2-pin plug, socket is identical shape:
http://www.plugsocketmuseum.nl/GB/BS372_5A-2p_plugs.jpg