This is the mechanism on the door that has to be closed first or opened last on a pair of uPVC French doors. I now know that this is called the slave gear box.
We had a quick look on the Internet but the videos we found only mentioned the first door not the second, so we called out a locksmith.
To cut a long job short, we were not impressed by the guy that came to fix the door. He took a very long time to fiddle with the door and eventually take it apart only to say he didn't have the part to fix it. I saw him take loads of measurements.
Eventually he struggled to put it back together and I suggested that as he needed to take it apart to fit the replacement part when it arrived, that he should simply close the door and use a screw to hold the locking mechanism fast. We could still use the first of the two doors to go in and out and the thing would still be fully secure when closed.
He contacted his office to put the order through for the necessary part and I had to pay for the call out.
From one photo I had already found the make and exact part of the bit that was broken and I could have had it delivered next day. There was no reason why a professional locksmith company should not have already got the part.
3 days latter I called up their office to find out when the replacement part would arrive.
They could not give me any information about the status of my order. I have no patience for this sort of thing and cancelled the order there and then.
This was 4pm on a Friday. By the middle of Saturday, I had received and fitted the part.
I now know that these locks are measured as the distance from the centre of the lock barrel to the front face of the lock and from the centre of the barrel to the centre of the handle spindle. In my case it is a 35-92 lock, 35mm and 92mm.
The little bit of knowledge I did not have at the start was that the overlapping plastic lip on the door was a complete unit simply held on by 4 long screws. Once that is off the slave mechanism is visible and easy to remove, just lots of normal screws.
Before removing anything, mark the exact positions of the metalwork for the latches. This will save a lot of time when re-assembling the door. In my case the locksmith and the original fitters had already put those marks on.
The lock barrel has to be removed. That is one screw going through the centre of the barrel assembly. Then the lock cam needs to be aligned with a bit of trial and error until the whole thing can slide out past the escutcheon.
Two screws from inside the house to undo the pair of handles and then the connecting shaft can be pulled out. That frees up the slave gearbox mechanism so it can be removed.
The bolts at top and bottom remain in the door, only a slide fixing needs to be removed to take off the flat geared connector rod assembly.
The lock came with the connector rods but they were a different pitch gear at each end, so I swapped over the rods from the old to the new gearbox.
Refitting the door is the reverse of taking it apart.
If you put back one connecting rod at a time, it is fairly easy. I started at the bottom, the slide fixing just sits over the end of the geared section preventing it coming out. A single screw stops the slide dropping down. I then did the same at the top. Took me a couple of minutes. That is the bit that the professional locksmith struggled with and I eventually told him not to bother!
At each stage I tested that everything moved the correct distances before proceeding to the next bit.
Job done, everything worked and I even made a quick adjustment to make it open and close a bit better.