Latest update. I deleted Myriad Pro from Font Book. I opened the last version of the file in Publisher and deleted about 50 text blocks with Myriad Pro - some with actual text, others blank. I saved as PDF and was then able to open in Designer - albeit with some changes to layers and the loss of pre-set slices - but at least I can access the file and was able to save a simple change.
It does appear that it was a font problem with certain fonts (Myriad Pro?) but it may also in part be due to it being a file that was originally created in Adobe Illustrator and then converted when I changed to Affinity. I've been able to work on it without any problem in Affinity 1 but something in Affinity 2 clashed big time.
It is exactly the same situation with another file that was originally done in Illustrator, worked fine in Affinity 1 but can't open or if it does open, I can't save in Affinity 2. I went through the same process - opening in Publisher, tidying up and deleting some Lucinda Grande fonts, saving as a PDF and re-opening in Affinity 2.
At least there is some progress and I know of a work around. I think it only affects files that were originally in Illustrator, then converted and updated annually for a repeat event.
I'll keep using Affinity 2 with fingers crossed.
Thanks for the suggestions.
Mike