I purchased a grade II listed building in the summer of 2003.  It was
a re-development of an old manor house into 4 dwellings in England,
UK.  It was supplied with an NHBC guarantee.
My neighbours condensing boiler vents onto my patio, which causes me
quite a nuisance due to the pluming.  The external wall through which
it vents horizontally is the boundary between our properties.

What were the building regulations at the time regarding the
installation of this type of flue, and is this therefore legal?

Are the same building regulations in force then in force now?

The flue discharging plume (which is harmless water vapour) could be a
Nuisance or a Trespass, which are matters of civil law.

This question was answered recently in the news:uk.d-i-y newsgroup, by
John Stumbles, at
http://groups-beta.google.com/group/uk.d-i-y/msg/99245578539e2f69?dmode=source&hl=en

" The boiler flue does not meet the regs: it is in violation of BS5440- 
(2000) and Building Regulations approved doc J since it should be no closer 
than 600mm from a boundary - certainly not _on_ the boundary and 
discharging over onto your side. "

There is a lot of useful information about boiler flues in the
news:uk.d-i-y Gas Fitting FAQ, at
http://www.makewrite.demon.co.uk/GasFitting.html
Scroll down to b) What are the rules for siting a boiler flue?

There may also be a pressure relief valve vent pipe near the boiler;
this will squirt boiling water in the event of a fault, and must be
positioned where that won't hurt anyone, and is usually turned back
against the wall.

Owain