Check out RHS website ... Quicker for me to copy and paste than explain myself ... "Medlars need annual pruning in winter to maintain a healthy shape encourage good flowering and fruiting. Prune as follows: For the first three or four years cut back the longest upwards growing shoots or leaders, of the main framework branches by about one-third of the previous summer’s growth, always pruning back to an outward-facing bud. Cut back to two or three buds from the point of origin, any badly placed shoots that cross over the centre. The aim is to achieve an open-centred, well-spaced branch framework. Leave short shoots unpruned but remove poorly placed or crowded longer branches ... Mature trees do not respond well to hard pruning ..." More available on the site.