When to prune a Quince tree.

Hi all - I have two Quince trees in the garden.They have been in for a couple of seasons now and are 3-4 year old. One tree has a really great shape (on the right) and full of developing fruit this year and the other younger tree (on the left) is very straggly and no fruit has set. When would the best time to prune it into a better shape be?

RHS says prune when dormant:
http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=298