Where possible just cut the cable close to the junction box and then fit the new head. Now you still have the original connections and can just match the colors. Blue = Neutral Brown = Thermostat Grey = Live Orange = Boiler
Operation is power supplied via the thermostat to the valve head motor on brown wire which opens the valve, this operates the micro switch which connects the grey live to the orange boiler supply.
With an unvented hotwater tank the expansion vessel is there for just one purpose and the giveaway is in the name, expansion. A volume of water in a sealed vessel expands upon heating, the expansion vessel is there to accomodate this excess volume of water which if it had no where to go would result in an increase in pressure which is why there is a PRV's , to stop the tank from rupturing. The expansion vessel on the heating side is there for the same reason, to allow the water in the rads to expand upon heating without causing a large rise in pressure, this is why you need the correct size or an oversized expansion vessel.
Not along ago I had a noise when my hotwater came on, a clanging sound which I identified as being caused by a volume of water trapped between the three way bypass valve and the hotwater zone valve. This water was at 70° C when the valves closed, it cools down and contracts so reducing the pressure in this section of pipework. Now when the valve re-opens you get the clang as the pressure equalises and was fixed by fitting a small expansion vessel in this pipework, a 0.5 litre did not work and it needed a 1 litre vessel which shows how large the expansion can be in a heating system.
