Thanks David.
That jpg from the files is a straight transfer of your drawing into the package, with only minor position adjustments, so all the credit goes to you for the original.
And for the curious, it was done in Sprint Layout 6, a very homebrewer friendly CAD program.
This is one of those perennial projects that crops up at regular intervals, so worthy of having that little extra done to keep it up to date.
Can you or anyone else recommend a free (and 'hassle free') viewer to enable the Gerber files to be viewed please?
Free I can do, hassle free, not so sure. They do need a little understanding of the Gerber format to get sensible output from. This is the one I use (free version):
https://www.pentalogix.com/t/software-products/viewmate
Frank.C - the gerber files contain the board outline layer that defines the size, which as David said, is 75 x 75mm (well, 74.9 x 74.9 if you want to be really really accurate).