Black Hole, you are the man. I did as you suggested and all now seems well. I am assuming then that the Humax Box and the Portta Switch were using IR 1 which was causing all the confusion. I can't thank you enough.
Not code set 1 exactly, that's unique to Humax, but sufficiently similar that there is an interaction. Having multiple HDRs isn't the only reason to need to change code sets - and if something else that interacts can't be changed, you have to change the one that can,
When you tell us about the fact that a button is not doing what you expect, it is vital that you also tell us what device button is pressed. Is the Hummy selected on soft keys when the hard keys control the Portta? If you do as BH says as then probably the codes for Left and Up will change for the Hummy and no longer control the Portta.
Not sure why your Portta switch seems to have taken over so many of the keys in your defined Activity. If it were me, I would delete the lot and start again this time using the correct code set for your Humax from the off.
Edit: Ah, I see it is being naughty and uses a code set that it shouldn't be and which clashes with the Humax code set 1.