I found another reference on t'internet, though this time for the Canon 200d, where a user had exactly the same problem as me. The usual (and expensive) suggestions were to use image processing software (aka the likes of Adobe) to correct this issue.
However, this time someone suggested turning OFF the in camera Chromatic Aberration. It's the first time I have seen this suggested.
I dived into the camera's menu system, found the 'Lens Aberration Correction' section and dutifully turned off Chromatic Aberration Correction'. Voila, no CA in wide angle shots. The setting didn't seem to affect zoom shots.
No doubt the camera is optimised for Canon lenses, and probably the newer ones at that. Hence the in camera processing. It is highly likely that cameras from other manufacturers may behave in the same manner, and offer a means of adjusting any in camera processing.
For me, this means no tedious post processing in Canon's Digital Photo Processing application. However, the application is pretty nifty, so I shall investigate it further.