Thanks for the advice.... not sure I'm any clearer but at least I know it's a grey area! We can't select our own boiler as the installation is under the warm front grant scheme, and none of the ones they provide are suitable for a through-the-roof flue for a house as tall as ours. So, think...
We live in a through-by-light terrace so all the back rooms in our house go directly onto neighbours garden, and vice versa. Our neighbour is more-than-happy for us to have the flue for our boiler exiting our utility room onto her land as this is what all the other houses on the street have, but...