Overhead Joins:
As VATCO has said, they're mandatory at Old Warden. They're also a useful way of spying the area before joining at other airfields.
The best published description I've yet seen for an Overhead Join is that shown in the CAA Safety Sense Leaflet 'Aerodrome Sense':
http://www.caa.co.uk/docs/33/SRG_GAD_SSL06.PDF
The other leaflets are quite good too:
http://www.caa.co.uk/publications/pu...e=sercat&id=21
Take a look - they're free!