**SOLVED**

I feel I should write a bit of info for anyone with similar problems and as clue-less as I was! haha. Well I removed the latch on both front doors (had to remove 5 bolts on door, remove black bracket, then unclup two 'rods' that come from the lock and the handle). I then checked them both, one needed its spring re-positioned.

This solved the intermittent fault of central locking not responding, it now works every time. The other problem was the passenger door opting out of the central locking...

This turned out to be the S-shaped electrical connector that carried cables from car body to the doors. Water had leaked into both of these over the past 15years and completely corroded one of the pins contained within.

Remove this with pliers, drill hole through the plug to accomodate new wiring, connect it in the door and inside car using crimp connectors and hey presto... Central locking working like new!

Took me hours of searching for relevant info so hope this helps ANYONE!