Just to let anyone who is curious know the outcome.
I contacted customer support, but was once again told ver 5 will not run on Mac m3 and the download in the members area is valid.
I accept that I can not use ver 5 on a m3. However I do not accept the ver 5 download is fully working. I managed to borrow a 2017 MacBook running os 12 Monterey. This should be fully compatible with ver 5. After downloading using both chrome and safari I still get the corrupted file error message. In the interim I decided to download Krita. What a pleasant surprise, it's really very good. It's early days yet but I think I shall be using it quite a lot.
Once again many thanks to all who tried to help me