Well I indulged in my rubber sniffing today and got nowhere. We cant find the puncture.... and it still goes down at the rate of 4psi in 2 hrs ?!
So we took the tyre off and checked it internally and found nothing and then refitted it.
Then checked the valve, checked the small slit on the surface again for any escaping air - nothing. So I took it back home and put it in the bath to look for bubbles, but absolutely nothing.

Now it's having an overnight stay in the tyre shop inflated to 60psi to see what it looks like in the morning.
The only thought is maybe it wasn't seating properly on the rim and removing/refitting may have cured it? Who knows. If it has goes down by the morning then I'll probably have to buy a new one as we cant find the puncture...
