So the guy has been and gone - removed an unnecessary splitter from when I had the TV service (hidden behind something, didn't know it was there!) and fitted a small attenuation device to the back of the Hitron.
The SNR for those two "problem channels" has now risen to acceptable levels:
Interestingly, prior to his visit, I had gone round and reseated all the connectors (minus the splitter I didn't know was there!) and even tried the Hitron on the connector where it comes in to the building. I noticed that the speed had dramatically improved when connected to the wall outlet. Moved it back to where it was originally, and it had returned to normal there as well. The changes the engineer made further improved it as well.
So it looks like checking everything and tightening it all plus his changes netting a substantial improvement. He did raise an eyebrow at the cable it connects to lol, as it's a custom bit of cable run to the network cupboard (not by VM) that connects to the end of the cable they did run. But he was happy with the quality of that cable and how it is terminated. Quite a friendly guy too, think all the engineers I've had here have been top guys.