existing stench pipe has been capped inside the extension with some air valve that lets air in but not the gases out? The smell is not nice and i need to get it vented outside but should this have been with the extension to conform to building regs?

reg, April 2009

Reason for being air vent is that your system is not been plumb correctly. you need expert knowledge.
it is capped because may be pumping over if air vent removed. not pleasent smell means there is sludge build up in system and your system may be not heating efficiant. Good luck

Mr K, April 2009