Morning everyone, firstly, thank you all for the thoughts/suggestions - really appreciate it.
Just a quick update, have been playing with the balancing, have managed to get 99% of my radiators hotter, and one of the two new replacements that’s being problematic, is also warmer.
The final radiator is being stubborn.
With everything else off, it gets red hot, with everything on, red hot at the top, lukewarm at the bottom.
Plumber is suggesting replace pipework at a cost of 1k, and that may fix it, and if it doesn’t he can do other pipes for another 1k, and that may fix it, but if it doesn’t, it could be the boiler pump or something else entirely.
Not keen on paying 1k for a “this might fix it, but might not”.
They worked before, now they don’t.
Have been nosing around, and the only difference they made when replacing, was the had to extend the pipes a little, as the new rads sit closer to the wall, so they cut the pipework, and extended it using plastic push fit/compression angle connectors, rather than the old copper pipe that just went straight up.
Could that be contributing to the problem?