[gottago]

Then it's a matter of the business case, what's to be gained by doing this? Does Freeview need more capacity? Do channels want to go HD? Is efficiency important, and could that be delivered in other ways (eg going to SFNs for the national multiplexes)

Certainly for the commercial muxes I can't see the business case. It's charge channels a premium for limited space on DTT versus charge much less for significantly more space in an age where the demand for DTT carriage is far lower than it used to be. SDN already makes ITV significantly less than it did 10 years ago - why significantly (I assume) invest in an HD switch when it will ultimately make them even less money?