A map showing the area immediately around St John's (including street names) is available here. The Postcode to use for SatNavs is AL5 1DJ. Useful information for those arriving by train or bus is here (this includes detailed instructions on the walk from the railway station); a map showing the Parish Boundary is available here.
If you are just visiting Harpenden for a Baptism, Wedding, Confirmation etc you will be arriving at:
A if you left the M1 at Junction 9.
B if you left the A1(M) at junction 4 and travelled through the centre of Wheathampstead.
C if you left the M1 at Junction 10, or are coming from Luton Airport.
D if you are coming from St Albans (including leaving the M25 at junctions 21 or 22).
A separate page contains useful information if you arriving by train or bus.