Using a combination of The AA website, and my own local knowledge, I came up with the following suggestion:
Take the A1 and the A1(M) to the M25. Go Eastbound to junction 24, which is the Potters Bar turn off. You need the first exit off the junction which will take you into Potters Bar. After about 1/2 mile, you'll be at a crossroads with traffic lights. There should be a pub on your left called The Lion, and the Canada Life building should be in front of you. Turn right here! You then want to turn left onto The Walk, which is about 200 yards further up. The ground is on your right.
Hopefully, this is the link to streetmap showing the last bit of the route from the M25 onwards: http://www.streetmap...p=newsearch.srf
For some reason, the arrow isn't pointing at the ground - the ground is at the top of the map.