SG4 8XX is the best postcode to use for a sat nav
From the B656
The postcode will bring you on to Bury Lane, a few hundred yards south of the church- you just follow the road round, go past Bury Farm (A big orchard on the right), and the church car park (with the parish centre at the end of it) is directly after it, on the right, opposite the bus stop.
From Bury Lane (the back roads)
If your satnav brings you in from Bury Lane, either via Old Knebworth or another back road, follow the road until you see the sign that says you're in Codicote, which is at the edge of the graveyard, the car park and parish centre are on the left shortly after.