They're not the simplest to change mainly due to the ancillaries that need removing to get to the belt but that applies to a lot of small cars where everything is crammed under the bonnet with little space, You should still be able to get belt. tensioners and waterpump done for around £300-350 if you shop around.
There area few "independants" round me that aren't cheap either which I put down to the fact they have offices and workshops that are as big and exotic as some of the main dealers but there are small "backstreet" workhops that do a great job for a lot less it's just a case of finding them
Try yellow, pages, Thompons and internet business searches to get some info andcall. Also worth giving a better idea of your location and see if any members can provide local intel
As AshOHFK said it is worthchecking with VW dealers as many keep the cost down and will price match genuine quotes to get the business