In the end, being not the best engineer, I decided to try the Hawke one piece adjustable mount...
It's a bit tricky zeroing in as it winds up and down on a screw thread that has movement in it - to secure the position there's a small grub screw (I'm not sure the mount would stand up to full bore shooting), but when you screw it down it makes about 3 foot difference to the POA at 200 yards!
Anyway, after about 40 rounds, I got it aligned and zeroed at 200 with enough elevation left on the turret to get to 300 and 400...
...though after what was happening to the .22 bullets in yesterday's fresh breeze at 200, I think 400 is going to be very 'interesting'...