So I cleaned up the barrel mounts with rasp and sand paper.
Then I cut a 1cm wide strip of .5mm plasticard, pre-bent it over a pen, and began to create the curved back of the barrel mounts.
Looks pretty good, but needs some sanding too.
While the curved plastic cured, I started with the turret itself. For this I drew some plans in CorelDraw and printed them, then using a rubber cement (temporary glue) to glue them to the plasticard and cut them.
And very quickly I got this basic turret.
Due to a mis-calculating in the plan there was a small gap in the front that I had to fill with a small stip of plasticard.
Then I did the storage bin for the turret rear (that will also mount my radar antenna - the winged item in the left front)