Good point.... ill certainly bear that in mind!!
No sorry thats me confusing matters, they are not changing the alternator. I misunderstood and thought the tensioner was a belt itself, but it of course isnt a belt which i know now.
They are replacing the Auxillery Belt, and I believe the tensioner (but definitely not the alternator).
Would you say from this noise/problem that the tensioner is broken/needs to be replaced? Or could the fact the alternator belt has slipped be the issue - meaning would JUST replacing the alternator belt and using the existing tensioner resolve this issue?
What I dont want is for the belt to be replaced, drive away thinking its fine, and then a few miles later the issue happens again. Would it be apparent when they replace the belt if the tensioner is busted?
I dont know enough to question what the garage are doing. They are a decent set of guys and I do trust them to some degree, but my own lack of knowledge worries me when discussing issues such as this with them.
Thanks for your help. And I agree I should be using an experienced VX220 garage, thats my impatience and the convenience of the garage! I am on holiday tomorrow and wanted the exhaust done before I went. Cheers.
The lump has been put in so many rep mobiles you can pick up a 2nd hand one for a few hundred quid from the bay. Lots of beer tokens and getting your hands a bit dirty would see it changed in a weekend with the help of people on here or pay a garage a shed load of labour. Also if you are in Kent I would suggest taking it to a specialist, there are pletny round there, and not a garage that has never seen one before. They have a few "characteristics" that make some jobs a right pain in the arse if you don't know what you're doing. Sure some Kent members will be along with recommendations soon 
Also, I am still confised? How much is the garage charging you for your alternator belt change? There is absolutely no need for them to be changing the alternator.
2 things to change;
Auxillery Belt (also know as alternator belt)
Tensioner