Glow-worm 40B system top-up and inhibitor issue?

My mother has a Glow-worm 40B boiler central heating system in a bungalow and i have recently added a radiator in her conservatory. the new radiator needs bleeding every time the system heats up and the hot water is now coloured and smells of the inhibitor i have added. I have tried to find an expansion tank to fill up the system but the only obvious way to do this is the the water tank which feeds the the emersion heater which is directly above it, I am at a loss as to how to top up the system as I cannot find an input tap or channel and I do not understand how the heating system has contaminated the hot water system.
Does the heating system share the hot water system feed and if it does how can I top up the whole heating system without affecting the hot water supply ?