Cheers Sprockets, making the cut post creating the sheet metal part with “Normal Cut” selected has worked. The “Normal Cut” option doesn’t appear unless it’s a sheet metal part, which makes sense.
The result is the cut edge when folded shows distortion, but when flat is dead straight.
Thanks to all the other suggestions, but the part is created in-context in as assembly, and also the strength/ attraction of SolidWorks sheetmetal is to model a part and then press a magic button to convert it to a flat sheet, rather than buggerising around with bend lines.