Hey everyone
Yep, so EC, my word to the wise is...replace the sodding thing now! It's taken several days to resolve this. There are several options depending on how knackered the actuator is.
Option 1: There is a round keyway on the side of the actuator (facing the outer skin of the door) about 1/3 to 1/5'th of the way up from the bottom where the actual bracket/lock is, a pair of grips that have sideways pointing tines, can be used to rotate the cylinder towards the door jamb engaging the mechanism and allowing you to open the door
Option 2: smash the entire thing to bits, you ideally need to make a hole behind the metal cover plate, (which you will have removed) which will allow you to flick open the latch internally, this is the option i've had to take as it fast became apparent, the engagement mechanism for the lock, would not move to the fully engaged position. (the manual cable to actuate the lock moved far too easily)
Once thats done, you can open the door and proceed with removing the lock, I'm currently on the removing from the exterior door handle part, but that should be relatively easy with a spanner or trim tool.
But please take my tale as a cautionary warning, if you think it's on it's way out, replace it, because i can assure you this has been an entirely unpleasant undertaking!