For completeness I am just reporting back.
piCore 9.0.3 installs and runs perfectly. without any issues on both CM3 Lite and CM3.
I have found it a fairly steep learning curve sorting out device trees, overlays and dt-blob.bin in order to describe the connected hardware, but that would be exactly the same with Raspian.
I think this combination of the CM3 and piCore make an excellent platform for embedded systems.
Many thanks to Bela and all who have worked to get piCore where it is today.
Tim.