slack belt or failed PS pump??

[20BOSH20]

Hi All

2005 multispace petrol 1.4

I've been getting screeching from the auxiliary belt when at or near full lock on steering. ignored it as busy, but have tested the belt and it's very slack, though looks in good condition. Anyway a week or two later and it's screeching pretty much any time i steer. has another look today and when i turn the steering wheel, the belt stops moving, hence the screeching. i have power steering nearly all the time except for the odd second here and there. 

Mr neighbour across the road who is a mechanic wandered over and reckons the PS pump is seizied and that's why the belt is  stopping. I find this odd, as i have PS all the time, so unlikely the pump is fooked. I can't quite work out why the PS pump pulley stops the belt when the pump is activated tho. 

can anyone explain?

[jj9]

If the belt is loose like you describe the belt tensioner can't be doing its job.

I think I would start by checking that the tensioner is working correctly.

.

[oilyrag]

Ignoring the slipping belt will cause it to overheat (hence the shriek) and glaze, so will not be good even when adjusted. Full lock will load the pump and the belt will slip. 
The lowest cost test will be to splash out on a new belt and fit it, correctly tensioned. Check the PS fluid too.

[Romahomepete]

Whilst the belt is poff for replacement check that the wheel on the pump turns easily.

Peter