prunus triloba - pruning advice

I have just taken over looking after a well established (15 years) standard.  It has been regularly pruned after flowering as recommended generally, but the centre is a mass of knotted thick stems.  Some are dead and arre clearly to be removed, but I think it probably needs considerable thinning to remain healthy.  Any advice on if, when and how to do this would be welcome.  Thank you, Helen