Can anyone explain what’s going on with the extractor fan I’m trying to fit.
My bathroom has a sloping roof so I’ve got limited access, but can get to the pull switch by lying on the stomach in the loft.
I’ve done as per the diagram below (all into one 6 pole junction box, earths omitted). Basically taken feeds off the wires as I can’t do it from the ceiling rose.

The bit that has got me completely flummoxed is that my ‘switched’ live is dead when the bathroom light is on and live when the bathroom light is off. I’d assumed that it was completing a circuit and thus when the light was off the switched live would be off, and when the light was on the switched live would be live. However it’s the other way round!

What have I done wrong (apart from getting an electrician in obviously!).

old electricians trick – lick all the wires to identify what’s live and what’s dead..

Your switched live sounds more like a switched neutral to me.

I think i’ve realised my retard mistake. My switched live was before the switch. Now seems to work. Drawing it made it obvious!

my ‘switched’ live is dead when the bathroom light is on and live when the bathroom light is off.

A conventional switch is essentially a break in a cable that that you can open (off) or close (on). If the light switch is on then current should flow through the switch, and both sides of the switch will be “live”. The switch should break the live feed before the lamp for safety.
It may be your description but consider asking an electrician to check things out