Wouldnt you descend to circuit height and join downwind rather than overhead join? If you were coming from south preumably youd be talking to farnborough and able to position accordingly. Its a long time since I have flown at blackbushe but joins from the west were usually taught to descend to the north and join midfield crosswind to middownwind / fit in to circuit traffic if I remember correctly. Dont recall overhead joins being used. Be interested to know what is currently regarded as the correct joining procedures.