R40MGF wrote:

Hopefully this photo should help give you a better idea of what you need to do.  

this is in the unlocked position. 
 

This is in the locked position. 

I've just had a bit of an Eureka moment!
 Looking back at R40MGF’s excellent and very helpful photos, while trying to depress the lock tab, I found that it wouldn’t depress fully (even using a screwdriver or similar) if the battery was disconnected if not fully charged. 
So I’d suggest to anyone experiencing similar issues, make sure the battery is connected and in tip top condition before you give up totally on your nicely lubricated lock not moving correctly.