Ian,
I tested this command on an AVANT with DVB-T, but the module is exactly the same as the one used in the Beovision 6-26.
Now, this is an interesting issue because another member had a problem where a different Beo5 command did not work for him on his unit and subsequently it turn out that the fault was caused by a marginal "IR" receiver in the TV. Yes, I know you are going to say, but all other commands work fine and it works for the Beo4. Like this other member he added delays without success and all other commands worked.
Therefore, I suggest you go along to your dealer and ask to test your Beo5 with one of their TV's with the DVB-T installed in it (but not the Beosystem 3 which uses a different DVB module)