Hi Malone. Immediately after making my post, I started to think I might just be trying to be too clever by half, and so it turns out to be. The longest of the 8 theme examples that I highlighted had eight letters, and I realised that I needed to highlight two additional letters at the end of this example. So without changing anything else in the grid I now have 8 examples, totalling 48 letters, with 2 overlaps, so that 46 cells have been highlighted.
I'm lucky that I hadn't actually posted it when I realised my mistake.
Serves me right for trying to be a smartass!
Thanks for your contribution.