Nothing wrong with Tee'ing off a single pipe system BUT there needs to be some distance between the tee's (normally the width of the rad), as such a system relies on the pressure drop between the two tee's (due to the pipe friction) to get the water to start circulating around the rad.
Perhaps something like this would be better. The S shape pipe retains the flow of the single pipe system if the rad is shut off and the length of the S pipe provides the distance between the tee's.
View attachment 172446
This may be where I'm going wrong. This, and the fact that I'm using a standard TRV and not one designed for single pipe systems (if indeed it is a single pipe system).
So, take a look at this picture, to see my incredible work for yourselves:
Here's some context:
Red is the "single pipe"
Blue goes to the flow TRV (the pipe leading into the TRV gets hot)
Green is the "return" which does not get hot
The dashes are where the pipe used to go to the old rad.
The reason it's so elaborate, is the two sets of pipes are on different levels. The green and blue going to the new rad are lower than the feed pipes.
I really hope I'm making sense here.
Hopefully, someone has the answer to my prayers, as I only have a week to sort this before I'm away for work.