New channels

As I was tuning the signal quality and strength showed up as good on all channels though just now looking at them via WebIf only channel 24 shows a good signal. I haven't actually watched anything yet but previously never had any issues with picture so maybe its just the WebIf display.
The tuning data accessed to create the WebIF display is static info from when the tuning occurred, and only retains the info for the last mux if tuned manually (or all of them if tuned by auto-scan).

The live tuning strength and quality are available on the i-plate (press "i", on-screen lower right) or better via Menu >> Settings >> System >> Signal Detection.
Unlikely. HD content is quite restricted (and encrypted)….I'd think BBC own most of their content for BBC3 & 4 so probably not an issue for them to bung them onto HD.

The BBC led the initiative to get recordings from the HD channels protected. DVD and download sales of popular BBC programmes bring in a useful revenue stream so they prefer to discourage people "time shifting for ever". The HD transmissions aren't encrypted, but a condition of the license for the use of the Huffman tables required to decode the EPG data is that recorded programmes have to be protected from being copied off the PVR. On the Humax this is achieved by encrypting the recording using a key held on the box (but there are ways to get around that - see the CF forum).
MikeSh is saying it is Unlikely that Film 4 will become Film 4 HD because they, (Film 4) don't own the copyright to the content they show, whereas the BBC do own the copyright to most of their content. The cost of getting a licence to transmit nothing but Hi-Def. movies would be too great