350 lwb 2001 Central locking problems

[Danger mouse]

Hi All, I am at my wits end here
I have several problems with my central locking and hope someone out there can assist me.
I have a 2001 350 lwb transit and the front doors (Passenger and drivers doors) have key facility, however, the side and rear doors do not.

My initial problem was with the lock on the rear doors had locked down and has never unlocked since. With this, the only way to open the side door was to use the remote which would unlock and lock almost immediately so I had to time it after pressing the remote to open the side door.
(To open the back doors I have taken the stared screws from the latch.)

My first question is... how do I enable the rear door to release the latch? (The slide to release button doesn't work btw).
Second & Third question is... Now the remote - central locking doesn't work I have to unlock the front doors manually and the side door isn't locked.
( note: the courtesy light works manually - but it doesn't work when you open the doors)
Is there a fuse? If so Where?

Many thanks