Ls live but Permanent L open

I have inherited a somewhat odd installation. I have a FSU on landing with just 3 wires connected to it (L,N and Lout). However Lout is not connected to anything in the junction box in which my fan is connected...

At the moment my fan and light are both switched together (with timed delay). I'd like to be able to switch the fan off so just the light works. My question is whether I can use the above mentioned switch on the permanent live connection, leaving the switched Live connection on the fan still connected to the light switch.

I know recommended practise would be to fit a three way isolation switch but would there be any problem with the plan above? (ie this would result in Ls being Live but L(perm) being open)

Also would this achieve my aim of stopping the fan working or would Ls still operate it (until the light is switched off).

Thanks

yes turning off the permanent live feed will stop most fans

however a proper 3 pole fan isolator is certainly the best way to go especailly if the bathroom has no window.

Thanks Plugwash! The bathroom has only a (large) skylight. So I might go for a pull 3 way switch I've seen mentioned in these forums as its probably better to do it the right way. (I think my fan - a 'Sector S Fan 90) could have been sold with an optional pull cord but not sure what would get switched inside - or if it would be possible to get anything to retro fit).

The real problem with the fan (why I want to be able to switch it off) with minimum hassle is its sooooo noisy (I've tried cleaning but no real difference). I can't creep to bed without it sounding like the evacuation from Saigon and hence waking my wife up... Maybe I'll get a new fan with ultralow dB, humidity sensor, PIR, toxic gas detection, minimal power consumption and and... its my own house so don't mind spending a few quid on a gadget.... and the fans (I've two similar bathrooms) are really bugging me as the kids always leave the lights+fan on (wasting precious heat being sucked out the house). I know I've wandered off the topic but anyone got any good recommendations for good fans they've used? Has the technology improved over the years? The blurb for the latest Vent-axia range looks pretty good.