Thanks Kevin.
Weird thing - just fired up Rstudio to check and the scroll bars are normal now.
All that's happened in the last few days is that I've installed the usual Ubuntu 18.04 updates and rebooted the machine. Not changed anything else.
Just tried changing the gnome tweak settings, suspending the laptop, plugging it into an external monitor etc - it still stays normal.
For info the rendering engine was on auto-detect.
So oddly this problem seems to have disappeared. I'll keep an eye on it and get back to this if it reappears.
Pete