DIY Doctor UK DIY Forums

In order to work properly, you have to have a permanent neutral, without a multimeter and know how to use it you will have to either put the original switch back or try trial and error, I suspect what you believe is the neutral wire is not the neutral wire, but as I mentioned without a meter...............

I fan isolating switch should be three pole like this one. Ie. 2 live ins and outs and one neutral in and out.
https://www.google.co.uk/url?sa=i&sourc ... 1862759221

If it has only one live in and live out then it is not really suitable for timer fan. You need two live supplies for a timer fan. One switched (usually supplied from the light switch or light fitting) and one continuous live feed. If you just switch one of the live wires then the isolator is not acting as an isolator but just acting as a junction box so hardly worth using it. If the fan is now staying on you have most likely put the continuous live to the switched live on the fan.
Hope you manage to get it sorted.

If the over run timer was working before you changed the switch then it must either be something you have done (wired incorrectly) or the overrun timer has failed at an awkward time) But as I said without a multi meter you can only guess, and guess..........