The belt was making a horrid 'croaking' type noise, not really slipping.
When I got the car it had just been fitted with a replacement second hand one, there's been a few bolts came out and became slack - so I replaced them. Today I was trying to cure the problem by changing to a geniune VAG belt and changing the fluid, but as soon as I got started that bit dropped out.
Luckily I noticed the bolt above the one which had snapped wasn't even there - so didn't bother changing the fluid or the belt, put a bolt in and done everything up and it seems miles better than it was originally - no croak, nowhine or squeal at all at the moment
I'll see how it goes and keep my eyes open for a new pump, but at the moment it seems much much better.