Squealing belt

Hi, I have Diesel turbo 5dr Lotus.
I am m experiencing a squealing belt particularly when engine is cold and/or in wet conditions, so generally on take off or under load. Backing off the accelerator immediately shuts it up and I have been nursing it by doing that and quietly taking it up through the gears. Once up and cruising it is sweet.
I suspect it is the alternator belt? The lower one of three, which is wider, feels a tad slack when pushing down on it.
Battery - having to jump start vehicle if engine is not turned over at least every second day. Although I do not know how old the battery is, if jump starting it always fires up first turn of the key. Suspect belt problem above the cause of battery not fully charging.
How difficult is it to adjust the tension of the belt? And where?
Looking from under the bonnet it appears the battery and battery box will have to be removed to access it? Or does it get more technical for this?
'Damsel in Distress'