I've done the 4 Counties ring, also from Stone! We did it anti clockwise (can't remember why!). As such, it doesn't really matter which way you go. However, I would go anticlockwise, so that when you are approaching Stoke from the north (which is only a day or so from Stone) you can decide if you have enough time to go up the Caldon canal (if you can, do go...scenery is lovely). As regards Harcastle Tunnel...it has subsided a lot due to mining subsidence, so the roof keeps encroaching, but its not a problem. It is one way at a time, but convoys are sorted out by the tunnel keepers. The other oddity is that it has no ventilation shafts. There is a fan at the south end to aid ventilation, but of course that would just suck in air through the south portal, so a door fits across the south tunnel mouth to prevent this. The practical upshot of all this is that if you are travelling south you think 'we must be getting near the end soon' and suddenly the tunnel kepers open the door to let you out!! Its a bit of a shock really! Have fun..its a great holiday.