We too have a 6foot bed and find every superking quilt cover I get is a different size! I think the measurements above (260X220) were the ones I got on checking google for sizes.
"Does anyone have an idea where good quality superking duvet covers can be purchased?"
Arnotts or their Bargain Basement Shop should be you first port of call. If they don't have them they might be able to send you in the right direction.