Clearly there is a problem with your Select. It seems that early Select units may have had this fault - at least Model Rail's unit did, and it's been suggested that the first Selects produced went into the sets and that those sold separately may be better.
I'd contact Hornby directly . Regardless of warranties, F5-8 should work and if they don't they you have some recourse. Added to which I get the strong impression much of the Hornby dealer network hasn't got a clue about DCC. My own local shop has been sellling the sets despite the fact that neither the proprietor nor his 2 regular assistants have ever handled any kind of DCC equipment themselves. If you approach Hornby directly , at least you'll be dealing with someone familiar with DCC and the Select
It's worth saying that the Select (and Elite) are designed to be Xpressnet systems , so in principle it should be possible to use the Select as an extra controller with a full Lenz system (not the Compact? but presumably including the Roco Multimaus) Mind you , in principle F5-8 on the Select work .
However a Select definitely cannot be used in connection with a system from Gaugemaster, NCE, or Digitrax (although for the record it should in theory be possible to link it to a ZTC system), and presumably the same will apply to Bachmann's Dynamis