Had a look around the drive belt side shock again and have some measurements from my car;

Outside of shock bracket on swingarm to fuel tank; 21mm

Belt edge to tank immediately aft of shock; 74mm

Belt edge to tank at top of sprocket (straight above wheel spindle); 77.5mm

Ohlins rear springs are marked as; 00180-65/62 L5115 and their OD is 66mm.

Could the issue actually be that the tank isn't mounted as far away from the centre line as it could be?. Haven't had a proper look at this so could be worth a look at it. Apparently Morgan's have sometimes been known to have minor variations between seemingly identical models; normally the differences are of course much bigger!
Doing my best at this New Year business. wine