Lee, does your car have full VW service history? If so, even though it is out of warranty, they should still cough up with a goodwill guesture, especially as there is a TSB regarding the dodgy air con compressors.
yes full vw history it is due a service but imreluctant to use them as im unsure how much it would cost?
Full VW history is excellent (for your cause).

Price-wise, VW UK instruct ALL their dealers to offer a price match. Go to a couple of other 'independents' within 5 miles of the Dub garage, get some written quotes (you MUST ensure that the work complies with the VW service schedule, and not some 'generic - one size fits all' service. You also need to ensure they only use genuine VW parts, and the correctly approved oil) - try to get them to separate parts and labour prices. Then wave cheapest said quote under the Dub stealers nose, and they'll match it.

I got quoted £650 buy them when I went in last time 

Jeeeze, was that JUST for a service, or did it include any 'extras'?

when you say good will jesture, do you mean they will fix it cheaper than the norm? or for free?
Goodwill can mean anything from VW UK paying for the repairs in full, to them say offering just 10% of the total price - to anything in between. But the simple fact that (a) you have FVWSH, and (b) this is an established and well known fault - there is a very high chance that VW UK will cover all the costs, or may ask you to pay say upto 30% of the bill. HAGGLE, HARD (but politely and civilly)!

HTH

BTW, when did your 3 year warranty run out?