Thanks for your help. I'm a little confused about the isolator. I don't have a timer on the fan so I'm not sure if that's needed. I've got the general idea though, and will try this out on Saturday when I get back home.
If no run on timer you will not need double pole switch or live supply.
Not sure if isolator switch is good idea or not as it will allow one to switch off the fan and whole idea is to force use of fan by connection to lights.
I know fans are required under building regulations and in a bathroom it will be part p. I was about to go out and if I tried to say more it would have delayed post. I would inquire as to requirements with LABC before wiring to make sure. Personally I fit isolator high on wall outside bathroom. I don't like pull switch isolators as in most cases hard to lock off and I can't see point if not lockable. Although in real terms the plastic clamps needed to lock off normally get lost anyway. Other option is fused isolator so removal of fuse has same result as locking off. There is not anything on outside of switch which will make you realize they are lockable normally have fan insignia on them.
All best Eric
There are two safe ways this can be done if your existing means of turning the bathroom light on i outside the bathroom then you can take a live and neutral feed down to a switch pattress by the side of the light switch and a switch wire and neutral th the fan.Then connecting the live and neutral feed to the incoming side of a switch fuse spur nad the switch wire and neutral to the fan on the fuse side of the sour and putting a 3amp fuse in this i an acceptable means of switching both light and fan on independantly. If only extending existing circuit no need to notify LABC.This only need be done in this cas if adding new circuit and not using existing
It is of course up to you if you inform building controls under Part P or not and I am sure many people don't bother but as to rules you are allowed to replace like for like without Part P and if your fan burnt out you could replace it but as soon as you modify a circuit by for example adding an isolator then it would come under Part P. There could be a gray area where the isolator is mounted outside the bathroom so I would suggest you follow links in projects section and down load and read yourself it is in layman's language. If you do decide to do under Part P there are forms without Internet version printed on them on IET website. Remember bathrooms are special locations. Also remember under new regulations ALL electrical items in a bathroom must be protected by an RCD although you are not required to alter existing if not being worked on once you do some alterations then you may have to fit an RCD if not already fitted. And a small job can snowball into a very big one.
Again it gets complex and if doing under LABC then I would let them tell you what needs fitting they are responsible for site health and safety so let them do what they are paid for. Be aware selling houses and in some cases claiming on insurance without Part P completion certificates can be problematic.
All best Eric