Swift Half wrote:
We have a chimney block that has 6 individual chimneys in it, which is
shared with next door. In our bedroom (actually on the ground floor)
there's an airbrick into a chimney that I thought was disused. That is
until our neighbour's boiler started up and I realise that there is a
flue running up the chimney for next door's boiler.

I would venture to suggest that this is not acceptable. What would
happen, for instance, should you decide to install a fire?