Low idle and sqeek/sqeel

Hi guys, I'm new to the forum and I'm looking for some advice.
I have a 51 plate Jimny and when my wife uses it for work approx 40 mile around trip the tick over during the day is steady but when she drives home at night with the lights on , heater on and radio on, on occasions when she comes to a halt the revs drop to 500rpm as though it's going to cut out but it doesn't. Any ideas??
I' ve cleaned and checked the PCV and the valve only allows you to blow through it one way and it rattles when you shake it.
Also when the lights etc are on there is a sqeeking/sqeeling from around the alternator area occasionally, it also does it sometimes without the lights etc on. The belt is fairly new and tight and it only seems to have started since we had the power steering belt changed about 2weeks ago. This belt is also tight. Any ideas??
Cheers
SORRY I MENT TO PUT THIS IN THE JIMNY TECHNICAL AREA

My 03 Jimny does the same thing. I find that the alternator squeal isn't too bad if you let it idle for a few seconds before turning on lights, wipers, heater etc. I had the belt changed and tightened, and it made no difference. I don't think there is a definitive fix, however if there is I would be interested to know myself!

As for the revs dropping, I find it is more likely to when cold or stopping suddenly. I increased the idle to try and ease the problem and that hasn't really helped, so I'm going to try and find another way to cure the issue. My Jimny did decide to drop the idle slow once that it cut, as I turned into junction. Cutting out caused the power steering to cut out, which unexpectedly made the steering very heavy, and turning became interesting to say the least. Hopefully somebody can shed some more light on the issues for you

The more electrical load means the engine works harder for the sane fuel at idle hence the revs drop. The squealing can mean the belt slipping. After belts are replaced they can sometimes bed in and might need a slight adjustment to compensate. Overtightening is to be avoided as that will knock the alternator bearings out.

The Idle Speed Controller should keep the revs constant. I would suspect hat its running too rich and the ISC can't add enough additional air or it is faulty or blocked.