Didz,
I normally take York with my local general and some hired mercs. Works very well. It lets me move the rest of my army down to Caernarvon for a T4 siege. For some reason by that time the English normally have a large stack of peasants heading that way that gets kinda bogged down if you sit in the castle.

Then when the English look like they are going to move on York I march an army near Nottingham. I do this until I've got Caernarvon working just well enough to keep my highlanders and Highland archers resupplied, then its bye bye England.