My original plan was simple DHW stat and TRV's on all rads, but this ,
I now see, has difficulties in achieving 'boiler interlock'

Not really. Interlock is easily achieved. Any argument is about whether it
is worth achieving or not. The benefits of implementing the interlock depend
largely on the attitudes of the occupier. If the occupier doesn't mind turn
it off and on manually, then the interlock is not as effective in increasing
efficiency as when they are not. I prefer a system I can just leave well
alone, yet works efficiently.

It is possible to have an all-TRV system, even subzoned for timing. You
simply plumb as S-Plan, with a 3 channel programmer, or with separate
programmers conveniently located. But rather than using the 2 port valve
microswitches, you achieve interlock using a flow switch after the bypass,
but before the heating zone valves. The output from the flow switch can be
combined with the HWC zone valve microswitch to provide a boiler call for
heat.

Personally, I prefer to use a programmable room stat, as it allows different
temperatures at different times of day. Having implemented it, it very
effectively maintains temperature in all rooms, including those with TRVs,
provided the system is reasonably balanced. The rooms with the programmable
thermostats are those used most frequently (master bedroom and lounge). This
provides very accurate temperature in those rooms (measured away from the
radiator) and a handy nearby override mechanism.

The only problem is the kitchen, which doesn't get to temperature. However,
this isn't really a control issue, as the room is 5m x 2.5m, with 2 external
solid walls, 3 large windows, 1 door, french doors and a single tiny
radiator at one end suitable only for a little bathroom. I'm planning to put
a fan convector (i.e. Myson Kickspace 800) in to resolve this.
Unfortunately, there's no room for insulation, although the draught proofing
could be seriously improved.

I cannot see an equivalent beast to the CM67 for DHW purposes....

Do you mean an RF cylinder thermostat? I'm sure I've seen one somewhere.
However, I prefer to use proper cable when possible. My experience with
other RF stuff is that it isn't quite as reliable as a bit of twin and
earth. OTOH, if running cable is very difficult, then they are tempting.

When wiring this system up, you'll still need a normal programmer. The hot
water side handles HWC timing. The heating side acts as a master controller
for your heating zones. You normally set this side to either "OFF" or "24H"
and allow the programmable thermostats to handle any timing. There is good
reason for maintaining a separate programmer. Firstly, you can turn off the
entire system simply, without traipsing round the house to all the room
stats. Secondly, wayward teenagers wanting 30C in the middle of the night
(and having a programmable stat in their room) can be defeated. It is best
to leave on 24H, if possible, though, as the programmable thermostat can
keep a reasonable overnight temperature, such as 12C, rather than turning
off completely. However, if you go overboard in balancing up TRV radiators
so as to starve the master room, this may not be as effective. Doing so just
a little, though, helps maintain steady temperatures.

Christian.