What sort of noise is it making?
Is it at low'ish revs when turning the steering wheel? and is it like a creaking noise?
If it is that noise it could be the clutched pulley (or just a loose belt) on the alternator (~£40) and you can DIY.
You say it wobbles like mad @ idle?
Assuming the ASV uses the clutched pulley on the alternator like the newer PD engines it's probably that, not the tensioner.
The replacement pulley for the alternator is about £40, new belt while you're doing it £20 and to put your mind at ease an idler £???