Mike how do you get onto Allerthorpe Common? We were down that way, found Allerthorpe Wood, parked in car park labelled Low Moor Wood, walked through the wood for a good while and came across a fence line with a sign saying Allerthorpe Common, but we couldn't find a gate?
Allerthorpe Common isn't that big so if you found the fence line, you can't have been too far from one of the two entrances.
I don't park in the main car park so I don't really know which side of the common you were on. Anyhow, below is how I reach Allerthorpe Common.
Driving down the A1079 (from York) take a right turn down the minor road directly opposite the Esso garage on the left (just before the left turn to Barnby Moor-B1246).
I think the minor road is signposted Thornton and Sutton upon Derwent. Drive down this road for about a mile (you will pass a chicken farm on the right).
You will see a small parking area on the left of the road. Park here and go through the gate signed Allerthorpe Woods. Walk down the path for approx. 100 yards and then turn left at the T-junction. Follow the path to a cross road and turn right. You should now be walking under pylons.
After a short while, go over the next cross road and just before the next pylon you will see a style on the right. This takes you onto the common.
There is a big fenced-off pond at the back of the common, with a marshy area to the left. I find that Adders favour the area of gorse to the right of the pond, or the vicinity to the left of the marsh. However, I have found them in most areas, including the paths leading to the Common itself.
Hope this helps. PM me if you want more information.