I got round to checking this today and for me at least the fix seems to have been fairly simple. The inner door handle (the one that sticks open) is fixed in place by one screw, remove this and lever the whole unit out, it's connected to the actual door lock by a metal rod so it won't come far out. If you look at the back of the unit you'll see where the rod attaches through a nylon 'hoop'. If you pull the whole thing forwards a bit and release you should hopefully see that it returns by itself meaning there's a working spring at the other end. Also on the back of the unit you'll see that the release handle is spring loaded, I applied grease to this whole area, spring, handle, anything that looked as though it moved and voila! it works! The handle doesn't quite go all the way back into place by itself but that could improve as the grease works its way through, if not I'll try again later but I do have a working door locking mechanism again.
I also (thanks to the god Aldi) have a fibre optic camera and used it to follow the metal rod back along the door, about halfway back it goes neatly through a plastic guide before bending down to head for the lock itself. It's possible that this plastic guide is also a source of friction so when the weather warms up I'm going to grease this a bit as well.
