Pruning silverbush/shrub (convolvulus

Hi I read on the RHS website that pruning of this is best in April I've clearly gone way past this so wonder if any one has any advice on when to do it - should I wait or do it before winter? At the moment it's in full flower and really bushy. Secondly does any one have advice about how to prune it - ( where on flower/stem) I'm a complete novice as you can tell!