My problem was it was trying to build UE4 not my project. Clicking on my project in the workspace view then compiling seems to have it working again (this is after going to 4.10.1 and having no change).
I still haven’t worked out how to build for iphone from xcode as the options seemed to be missing but can launch successfully from unreal, so until I have an iphone specific problem to debug I’m putting my head back in the sand.