Hi all,
Just thought I'd throw a quick resolution message here. Seems that ArcMap desktop and ArcPro don't always play nice and when you add other packages in there it becomes more fun. The ECW file that I have used for ages and which was the primary one which threw the error, has had an AUX and an XML file created for it by one package or other and ArcPro didn't like them. I deleted them and it opened it happily. I found this out by opening the original ECW in ERDAS Imagine and exporting it to a different ECW file in another folder. This new ECW of the same image, opened fine in ArcPro.
So, I should have realised that, given the pain we've had with AUX files in the past, that this was a likely issue. Blow away the unnecessary files which will be regenerated and it may work. Of course, next time I try and open it in ArcMap Desktop it will probably have a dummy spit!
Cheers all!