Rod – have a look at Toolstation under the ductwork – think it's buried in the "electrical" section! They do a 100mm straight duct – duct coupler, but easy to fit to a box as you'll have to cut a hole in the box side anyway, just insert the coupler to leave a spigot sticking out, retain in place with 3 or 4 small right angle brackets and seal any remaining gap with bathroom silicone or decorators mastic. The normal 4 inch dust extract ducting should just push straight on to the the spigot. That's what we've used in the theatre workshop on the table saw.
Keith