Rightly or wrongly I had rather given up on getting an up-to-date GIMP on Ubuntu 16.04LTS. In fact GIMP 2.10.10 (7 Apr 2019) is available in the official “store” - “Ubuntu Software” icon. Thank you to snapcraft!
Indeed, you can also install flatpak and install it that way as well.
I will also highly recommend Andrea Ferreros GIMP AppImages. These are less restrictive than snap or flatpack packages.