Hello Ben-gee. Many thanks for your interest and your advice.
1. You're right about the two cartridges looking different lengths in the picture and, again you're right about it being due to the rotation of the 'cogs'. When both cartridges are brought to their minimum length, they are exactly the same length below the 'waist'.
2. You're also right about the bottom O-ring. As a test, I removed it completely and offered up the cartridge to its housing. It went in perfectly - as easy as pie.
3. I then greased the O-ring and replaced it on the unit. When offered up to the housing, it wouldn't go in fully (of course) but I did what you suggested with the three bolts and that worked a treat. I then removed the cartridge, added the rear unit (spring and temp sensor) and put the full assembly back in to the housing. It looked perfect and I was very happy indeed – until I turned the mains water back on and found that the shower just wouldn't turn off. No matter what I did with the flow handle, the water simply continued to pour out at full flow. Do you know why this happened?
4. I removed the cartridge, spring and temp sensor, then put the cartridge alone back into its housing and, when I turned the mains water on, the flow handle worked perfectly. The only problem was the shower would not get warm.
5. I then tried again to install the full assembly (cartridge, spring unit and temp sensor) but, since my first success in getting the whole assembly into the housing, I have not been able to get the cartridge anywhere near the bolt holes with the spring unit and temp sensor installed at the back of the housing. Also, even when offering up just the cartridge (with the O-ring in place) it will now not go all the way in and, even using the three bolts method', I can only get it to within about 1/4 of an inch of being fully flush with the face of the housing - at which point, the bolts will not tighten any more and the cartridge will not move any closer to the face of the housing.
6. As I write these words, I have the cartridge minus bottom O-ring, spring unit and temp sensor, installed in my shower. That allows me to turn the mains water back on and, as a bonus, the showerhead isn't dripping. Sadly, there's no chance of a warm shower unless I get back on the job and somehow make it work. ;(