Thanks again folks.
I've done a bit more research including watching this excellent video...
I think the couple who make these vids deserve a special mention - often when watching youtube videos for education I find myself underwhelmed by a lack of information / depth; however every time I watch content from this channel I'm pleasantly surprised by the quality of information offered and rarely find myself with any questions left by the end
In summary it looks like the Marathon Racers are potentially the sweet spot between all the main factors (grip, cost, puncture protection, mass, rolling resistance..) while I have no gripes so far with my experience of them so I'll probably stick with these. The Conti urban seem close in many ways but I'm happy to stick to a known quantity; plus it's nice to have matched tyres on the bike.
I do like the look of the tan wall folding variant, but I'm not sure that they're worth the extra (getting on for double the price of the standard rigid offerings) and again the bike would probably look a bit cack with one of each fitted while the remaining black item wore out..