So I spoke to the person at the shop, explained how I'd come to first think there were 5,500 previous shutter releases (due to using unformatted card), and then found with EOS Count there had been 1,600 or so in fact.
He wasn't at all surprised (so at least we didn't have the fake "not when it left the shop" arguement, that could have occurred), and suggested that he had mentioned that it was an ex-demo. When I said not so, and clearly I wouldn't have paid full retail price for an ex-demo, the response was that I got a SD memory card chucked in. My view was that was cos I spent another £1,600 on other things when I brought the kit ... blah blah.
Anyway, he immediately said he had sealed boxed ones in, and if I wanted to swap it I could. I would need to pay for the card, which was fine.
I swapped it as soon as I could get there (no kidding!!), and he actually did NOT charge me for the card.
So, I feel like I have had a lucky escape, and I'm clearly glad that I brought from a shop so I could easily follow up with any issues quite simply.
I don't think it was the best purchasing experience ever, but I would say that I was very happy with how they resolved my issue when I contacted them.
Thanks very much for the invaluable help you've provided, in guiding me through this experience. It's very helpful to know other folks are prepared to take the time to help out.
Thanks, Simon