Am just cleaning the window corner steel as I made them remove the ply etc after finding the frame leaked rain onto the unbitumened steel. There was a nasty rust place about 5cm2 below the cill, so I feel vindicated. From previous reading, I've cleaned the mastic off the steel, but regarded the rust as sacrificial and reapplied 2coats of FeO (rather than hacking it off), and am now on to 2 coats of bitumen. 

Should I put a bit of sand on bitumen onthe surface of the steel below the DPM? It needs to be encased in concrete. It is half encased already so I hope it's OK. This part (ie below the cill) *is sitting on the slab, below the DPM and there isn't anything under the steel. 

Any tips gratefully received. 

No need to sand - this was done on flat roofs to toughen up the surface and prevent stuff sticking to it .... birds feet etc.