I found the solution to this in the end, turned out to be Heat Creep. Like a prat, I had installed the cooling fan duct upside down onto the heat sink, so the throat tube was getting hot higher up the tube that it should do. In my defence, I hadn't noticed that in the installation instructions they turned the assembly upside down in the picture, so I was sure I had the fan the right way up! Well I flipped it over and the prints were better, but still not reliable. Got rid of the rubbish fan that was either supplied or was on the original CR10s, and put a Noctua fan on there instead with the PetsFang mod, also adding the blower, and not had an issue since!

I must say, I learned a lot about the technicalities of how the printer actually works whilst trying to resolve this problem. Oh, and the cheapo PLA has also worked fine since making the swap to the new fan and PetsFang