Ok, after 24 hours of searching and very nearly reinstalling i found a couple of posts which i have linked below.
The remedy was simple when you know. It was the ~/.config/kwinrc file that was causing the problem. It seems that after system updates it has been known to do this, causing all sorts of problems. I renamed it, rebooted, sorted out the settings that had now been set back to default, and it was all working again! Phew, that was close ^^.