Thanks for the words of support James !
I’ve posted the same question over on the Adobe Premiere forums too, but no bites on that line either. ;-(
So at the moment I’m just stuck with a bunch of files, nicely edited and ready to go and pretty much left to twiddling my thumbs having experimented (in a controlled manner) with every config parameter I could think of.
I don’t think its a hardware problem with the C300 used, because I’ve had no problems with a couple of 1k clips shot on the same camera before 4k was enabled.
Its just weird. I’m starting to think perhaps its not me but some weird bug ? But then surely someone else either here or on the Adobe forum would have already piped up “yeah, I’ve seen that one“.