I have now received my new Hornby HM7040 Legacy Dongle and plugged it into the X-BUS socket on my Roco Z21 DCC controller.
I can report that it seems to work fine in that I can access and control my older standard DCC locomotives via the Hornby HM DCC app as well as the ones that I have with Hornby HM7000 decoders. The App remembers multiple locos as separate entities via the dongle/Z21. Interestingly the speed control etc. on the Z21 change in real time when changed on the HMCC app and vice versa.
So far so good then. Well done Hornby!