jonnyseven – I had a similar creak and tried everything. Mine was creaking when you push on the pedals (even with feet out of the cleats). I went through the s/post, BB cleaning (even replaced), etc. Ended up swapping out the whole rear wheel and the creak stopped…
… tried sticking my original skewer back on and still no creak so knew it was something on the wheel which is a Hope Pro II on a DT Swiss 5.1D. Removed the cassette, greased the freehub body (not sure if you're supposed to do this), the whacked the cassette back on and tightened it up to death. The creak has improved massively.
Search for Hope Pro II creaking topics and you'll find plenty, they might also offer some help.
Hope this helps, let us know how you get on.