Can supply heat for both depends on your particular system.
Expansion vessel for system, means you have a sealed system.
WCF means you have an open flued appliance which needs regular service.
Could be lime scale in plate heat exchanger, or filter on cold inlet pipe.
But i would try turning stop cock full on first in case it's nothing to do with boiler.
Service is as MI.
What is the pipe made of ? if it is old earthenware (glazed clay pipe) you can get a plastic fitting which provided that the pipe is straight will go over the earthenware, they do these earthenware one end to plastic at the other end.
No valve of any kind in open vent pipe.
If your cold water tank is higher than your cylinder it will work on gravity regardlest of it being lowered as you said you will just get less pressure,
if you have pumped supply then you can have problems.
If you have a Honeywell 3 port valve this opens for mid position and heat only by the sychron motor to work micro switch, the valve doesn't do anything for hot water thats why it works for hot water but not for heating SYCHRON MOTOR i think you find.