On kappers' suggestion from Sunday and the low-slung-shorts-exposed-back issue, I dug out some bibshorts to try out this morning. Thing is, they're 'Large', and I do believe I'm a bit tall for that - let's just say my... shoulders... were tight. But. Warm back, and the issue is definitely more down to the shorts riding low than the windcheetah riding up.
Must dig out belt...