Will a himidistat go off during the night a lot?

They are adjustable anyway, but IME they don't usually trigger unexpectedly. You may find you need to tweak the setting as the season changes a little.

The message from John Rumm contains these words:

As those with good memories may recall I took advice last year on a suitable humidistat to control the window fan in my sisters downstairs bathroom (Chalet type bungalow with loft conversion).

Having installed said humidistat I set it at the lowest possible setting compatible with normally off during daytime. There were one or two instances of unexpected on during the first few days so a bit more margin was allowed. ISTR that it ended up being tweeked about 5% and is now set at about 70% but one or both of these figures could be wrong. It now functions only at bath/shower times and my sister and brother-in-law are pleased with the result.

I don't have a good memory which is why I had forgotten till now that they were so impressed that I now have to swop the timer on the bathroom fan upstairs for a humidistat or junk the existing ducted fan and get a humidistat controlled one. Thanks for reminding me.:-(

You can add a humidistat to the existing ducted fan - save junking it...

The message from John Rumm contains these words:

I am 250 miles away at the moment and my memory being what it is I can't remember the details of the timer. I need to be able to disable the timer.

ISTR that some of the advice I received before was on the lines of buy a cheap humidistat controlled fan and cannibalise. :-)

The standalone humidistat that TLC does is quite pricey IIRC, but they do work well IME. The timer fan will require a permanent live connection for the timer to function. If instead you connect both switched and permanent lives to the output of the stat, it won't be able to run on!

The message from John Rumm contains these words:

It was your advice I took last time as well.

Humidistat from TLC cost me about £40 last December.