Jack wrote:

HI,

Does anyone know what the regulations are (eg., Gas Safety, British
Standards) for the amount of clearance required for a balanced flue?
Our neighbour has property which borders our house and is intending to
shift his fence forward (along our house) to gain more room in his
back yard, but that would mean as he moves his fence forward, that
fence will be closer to our flue (this is the flue from a gas fire
approx 3-4 kW). I wasn't sure how much room we should technically
leave there, and want to ensure the right amount of space is left in
and around the whole general area.

1.2m from a boundary IIRC, and if you have less it's your problem, not his:
it's up to you (or your installer) to allow this space from the boundary so
that if the neighbour e.g. builds an extansion up to the boundary with a
window or air vent directly opposite your flue, the flue will not discharge
into their house