Hmmmmmmm, lots of very interesting posts here.
Immediate problem solved as far as having two tarps go - the smaller of the two - the Highlander rip stop nylon one - has been commandeered by the kids (after a fun afternoon playing with a new pop up tent and the tarp extension).
So, it seems the British army one is not as popular as perhaps I thought it might be (I wonder if that's because of the camo print).
The DD tarps look good - simple, strong, and green. The others look good too, but I imagine are owned by those who use tarps a lot and know what they like in a tarp and have tracked down what they're after.
I'm sticking to the the British army one for now. After sleeping on it (or should that be under it?) I don't much see the point in buying another (and another and another), without seeing it first, only to find out it's not quite right in some way, as I suspect it might be. The British army one is strong, simple, about half as heavy again as some of the others admittedly, but then I don't carry it around that much. I think that for what I use it for and for how much I use it it's absolutely fine. And for that reason I don't need another.