Thanks to all, I am planning more ground based photo work. There are times when I feel the need for an increase in the Wow factor and 28mm is just not wide enough. Also I will be working with large structures such as ships,cranes,docks. (already done the air views of the location)
So its looking like the Sigma 10...20 which includes the equal to my original 28 mm plus the ability to go even wider.
While I am not  fixated on believing that a pro photographer has to have lots of kit , I do believe that I need to keep re investing  some of the profits into expanding what I have as tools of my trade. I aim to deliver exciting and unusual images and not just from the air !