Adair, you know a lot about Pinus. What would you do?
Would repotting be too stressful now? It's the right time but still, it's a weak tree.
Why would you get it repotted? It's not hard to do yourself. It's easy to find how to do it online.
This is a very sick, weak, tree. If it’s not dead already.
From the text of the OP, it would appear that he purchased this tree, but he appears to be new to bonsai.
Looking at the picture when it was still on the slab, it appears to have some needlecast or something. I see some yellowing needles.
I suspect that the OP was advised to have it repotted at the time of purchase, which would have been back in September or October? Not the optimal time to repot.
And it appears that it was just moved off the slab into a pot. Was any real root work done? There’s no way to tell. The soil of the original root ball goes appear to be very heavy in organics, while the new soil is very open. That’s a recipe for disaster!
What “should” have been done would have been to do a “half bare root repot” to transition the roots into the new soil. It doesn’t look like it was.
So, what happens is the new soil is SO fast draining when compared to the old soil is water drains out before the old soil (where the roots are) can absorb any water. The roots, meanwhile, continue to suck all the water out of the old soil. The old rootball gets dryer and dryer. Even though it might get watered!
Have you ever seen a kitchen sponge get do dry that when you pour water on it that the water just beads up? That might of been what happened.
Moss on the surface can also repel water.
So water hitting the moss runs off to the edges where it runs thru the new soil and out the bottom.
On the other hand, the tree might have had a festering case of needlecast, and THAT’s what has caused the decline. I really can’t tell from just looking at the pictures.
Anything I say is just a guess.
I know the UK has wet winters. All that stuff I wrote about having a dry center rootball may be totally wrong! It may have been festering root rot! From being too wet! Again, a Half Bare Root repot might have saved the tree. It’s virtually impossible to overwater a tree in good inorganic bonsai soil.
So... my advice IS to take it to someone else to assess which of the above scenarios might have caused this tree to decline so badly, and then figure out if there is a path to recovery. I fear it may be too late.