The cheapest and more practical is what is suggested: Alpha pump,
by-pass, flowswitch and TRVs all around. Then split the system
into two zones,

Obviously, the system would have been overspecified, but it was an example
of the "best" rather than "best value". The system I'm just doing myself
will be converted to up and down zones with a 2 port valve and programmable
controller each in the lounge and master bedroom. The other rooms will have
to fend for themselves on TRVs.

A system like that is pretty cheap, easy to install (or even modify an
existing system to) and allows separate timing for the sleeping and living
room areas as a whole, which is quite an advantage when the time usage
profiles are so different between the two. The differences between 2
bedrooms, though, aren't quite so great, so might not justify the provision
of additional zoning.

BTW, how does the Alpha know when to restart? Does it "poll" at a certain
frequency to check the resistance?

Christian.