No probs Roger,

I'm overdue a visit up to Old Hartley, I'll get you some more info on that car park/beer garden stone and shall mail it to you.

Looking at the photo, I've just put two and two together and realised that the strange 'cracked' effect it displays indicates that it might not be sandstone, but could be a lump of this annoying type of stone that is to be found lying about all over the area.
On the surface, it looks like sandstone, but that's just an oxidised veneer of something much harder (and heavier). I know this having once carved a lump of it and got a nasty surprise when I whacked it with a pick. The reason that bits are left lying about Tyneside is that it's deceptively heavy, and unsuitable for building (though it does have excellent thermal properties).

The 'half-peeled egg' appearance of that stone would fit with it having been lifted and dropped onto a hard surface, removing some of the veneer. Interesting. Maybe it is the original stone. In which case Mr Horne nearly did his back in for nowt ;)