I probably should have included my build platform details which is Ubuntu 20.04 (running on a VirtualBox VM under Windows 10). As it turns out, this was completely relevant it seems…
I switched to another VM running Ubuntu 14.04, installed mlinux v5.3.5 again via git clone, followed the same setup steps as before and this time the mlinux-base-image build ran to successful completion. So happy days
I didnt see any reference to Ubuntu versions in the mlinux getting started guide, but perhaps it is detailed somewhere else…