I'm taking the easy way out and returning the cylinder for a copper one with shower take off (with dip tube) pre-installed. It's £50 cheaper and doesn't require either my bodging or mackling skills.
Thanks for your comments.
The tank is a Gledhill Stainless Lite, it's the same cylinder as the unvented except there is a stop end in the tank where the temperature and pressure relief valve tapping is. Their literature says 22mm compression fittings.
Having posted I then noticed an identical thread I had missed...
I am fitting a stainless steel replacement indirect open vented HW cylinder, the outlet on the top of the cylinder is a 22mm compression fitting (as are all the other inlets and outlets). I want to fit a "shower flange", however I can only find flanges that fit to a 3/4 or 1 inch outlet...