Thanks Carl

Not sure I completely understand your advice..
What I was getting at was : 'DVD' (on MSR) is associated with 800. 'DVD' is not associated with HD621 in any way - ie literally HD621 does not use the DVD (MSR) button so - why is it in the HD621 setup in MConfig?
When I try and delete DVD from HD621 in Mconfig, it immediately says 'do you want to remove DVD' and if I do, it removes the separate 800v4 entry completely - which is obviously bad.
The 'Last Valid' options under DVD on HD621 are greyed out, so I can't alter them anyway. (To H1, H2 etc as you suggest but I don't want to point 'DVD' to HDMI anyway because DVD is my 800 on MSR and has nothing to do with HD621).
Hope that clarifies my question.
The (physical) sources of HDMI that I have being PS3 and NVIDIA Shield network streamer are mapped to H6 and H3 and work fine ,(on MD1 3xSPDIF).
With the above in mind are you able to reclarify your advice please?
Thanking you

Andrew