Second question.

What is the physical make up of the back boiler? There are 4 tappings -
two for the hot water circuit and two for the heating circuit - are they
"commoned" inside" (it is not clear from any drawings or explanation).

The four outlets are all commoned to the one water jacket.
There should be two near the top and two near the bottom of the jacket.

On my Bont ESSE Model Four I used the left hand two for the indirect
heating loop in the hot water cylinder. The top outlet to go to the top
of the heating loop and the bottom outlet of the jacket to the bottom
of the heating loop. The right hand pair of outlets were used for the
central heating (CH) circuit. The top (Hot) outlet had the circulation
pump fitted to it so that the pump pushed the hot water out to the
radiator circuit. The return (Cold) pipe went into the lower outlet of
the water jacket.

The cold water feed from your small 4 gallon header tank can connect
to the same pipe as goes to the bottom of the water jacket or the
bottom of the indirect heating coil.

My 'Leak' or 'Dump' radiator, which is not pumped, is on the same
level at the Bont ESSE. The cylinder is one floor up from the Bont
ESSE. The 'dump' radiator is effectively across the indirect heating
coil except in my case the cold, return from this 'dump' radiator,
returned to the pumped return pipe of the CH system. The reason for
this was because the cooler water output of the 'dump' radiator will
not go back up-hill to the indirect coil as that is virtually where it
came from in terms of water height.

Sit down and draw it. Remember the priciple of a 'U' tube, you can
NOT have a radiator higher than your 4 gallon header tank.

Chris.