I'll try and respond to your various comments.
Photos - no need. Suffice it to say that of the 16 pipes, possibly two aren't like bananas. System was installed when house was new, so no excuses. Calling him a Muppet was kind.
Various comments forgetting that appear to ignore what I'm asking.
"Answers" were found, in the case of the YouTube clip, after a comment by someone on here. I'd already seen
http://copperplumbing.org.uk/sites/default/files/content_attachments/jointing-capillary_0.pdf but as I've never tried it, wanted to get feedback from someone who had.
Compression fitting - I assume you mean compression reducers. Tried that and can't find any locally that fit the TRV.
Getting water out of a near vertical pipe on a drained system isn't difficult.
Of the three new houses/new central heating systems I've had, all have come with faults, the worst of which were gas leaks on two. I found the first on the first system I had - a nice wee flame on a coupling. After fixing that, I leak checked the whole system, and did the same on the other two systems, one of which had a gas leak. The Muppet(s) who installed the current system couldn't even get the pump right. They had four ways to install it, three of which were ok. They chose the fourth, so the pump failed due to water ingress when the pump had been bled.
A friend suspected a gas leak on their sitting room fire. Whole family were getting headaches and there was a slight smell. Called out the local heating engineer (?) twice. Nothing found. Asked me to check. Found leak in less than 10 seconds, isolated fire, and got them to call out the engineer (?) again to fix the leak I'd marked.
Now the purpose of this isn't to denigrate anybody in general, though certainly some in particular. What this illustrates is that the "professional" (so called) don't always get it right. It's not difficult to check for gas leaks with the right equipment. I have no idea what the individuals concerned had access to or used. I had a Gascoseeker, and knew how to use it.
I came here, asked a simple question, and have received a lot of flak and little meaningful assistance.
I trust you're not charging time spent on this forum to your customers.