Thanks for your help on this Jacob but I'm 99.9 % sure I found the exact problem tonight. I actually started at the wrong end of the circuit .. 

Turned it on and noticed the translite looked a little dark . On closer inspection the bottom half of the GI lights behind the translite were out . Thought it had blown a fuse so went checking . Pulled some wires out the way a bit and noticed that the connector at J120 has been damaged at some point . There is 1 or 2 wires that are now soldered directly onto the pins ..  . Not only that most / all of the wires coming from J120 have been cut / re joined and some of these joins must be a bit dodgy ( hence the intermittent door problem ) .

Bloody hackers !! 

Oh well at least I know the cause of the problem now so should be all good soon .