Stephen wrote:

I understand with conventional flues you have to be careful with
extractor fans. The brochure says the fans have to be installed to
BSxxxx. We have a fan and a cooker hood so what does BS xxxx involve?
Where can I find and read it?

I presume the xxxx part is a number. I suspect installing the fan to the
BS is not the issue, except that it will probably tell you to look up
something else (often the manufacturer's information) if there are any
open-flued appliances in the vicinity.

The guidance in Approved Document J is, "For oil appliances: limit fan
capacities as described in OFTEC Technical Information Note TI/112 and
then carry out flue draught interference tests as described in TI/112".
So a copy of that OFTEC note may be your best bet.