Magnolia stellata, between midsummer and autumn (or it bleeds); Winter jasmine, immediately after the main flowering in early spring, when you can cut it back really hard; sambucus, depends whether you want flowers or not, but I do that any time it gets out of hand. Cobnut tree, can you be more specific - do you mean Corylus contorta, or another ornamental version, or a proper tree grown for its nuts?

Thanks Bamboo. Have no idea what kind of cobnut tree it is, it is has self seeded though. Is that of any help?

The RHS has a page on cobnuts and filberts - they're slightly different from one another, but the page also includes information on pruning and training. Google 'cobnut tree' and that page should come up as one of the options, otherwise, go to the RHS site and search for it there.