Fan belt issues!!

Evening All.
Replaced my fan belt recently due to some horrible squealing and being very slack.
the new belt doesn’t seem to of fixed it, I tension it as much as I can and its great for 100 miles max. Then belt goes slack and squeaking reappears at high revs.

Is this common with these? What am I doing wrong/ any suggestions? Wonder if I need a new alternator?
Many thanks G

Disco or defender engine?
Def engine the tension lasts a while, disco is a very poor set up with the worlds shortest belt.

If def engine what width belt are you using?

You also need to check the alternator mounts as well, as it not unusual for the bolt/s to come loose, the alt mounting bracket to engine bolts also need checking.

Defender engine. Belt pic is attached. Although I’ve started it up this morning, drove round the block and the thing snapped. Somethings not right!