Render engine reverts to Blender Internal after it completes a render in cycles

Bit of a weird one, does anyone know why it’s started to do this? Just checked - seems related to a particular .blend file, well a few to be precise, all various iterations of the same file.

Are you using multiple scenes in the compositor that use both renderers

Nope, and also in a further plot twist - the same blend file on another machine doesn’t switch render when it finishes…