Bron, I made this today - used baker's flour and added baking powder, used milo instead of Nesquik and ended up with a wonderful cake that wasn't quite cooked in the middle so came out tasting more like a mudcake than a sponge. It was much denser than yours appears to be and I think with the Milo was a tad on the sweet side. Will have to try it again using normal SR or plain flour (with baking powder added) and Nesquik or a drinking chocolate powder that is finer than Milo. Despite not being entirely cooked through it came out of the tin in tact and had a nice crusty type top.
Just out of interest - did you sift your flour three times like it is suggested for sponge cakes?