Plot: In the Beechgrove Garden, Jim and Carole take the fortieth anniversary celebrations to a whole new level as they unearth the harvest of ruby-coloured potatoes planted 13 weeks ago. George and Carole revamp the rhododendron border at Beechgrove, and they show how to 'air-layer' a big shrub, and how to lift the crown of a small tree to ease congestion in the border