Sussed it. You have to move the unallocated space so that it is physically next to the C drive. Choose 'resize/move ' D and expand D into the unallocated space. Now use the slider on the other end of D to create as much unallocated space as you want to increase C by on the left hand side of the bar representing D. OK that screen. Now you'll see the unallocated space between C and D. Expand C into the new unallocated space. OK then Apply. Your machine will shut down in order to complete before booting into Windows.