Can anyone offer me some advice?

My brakes have failed 3 times now all when doing no more than 15 mph and changing down at junction/traffic lights.
Took it to local VW dealership after second failing they said they had bled brakes and all was fine.
Brakes failed a third time on 11.9.08 called VW assistance who sent RAC who said it was ABS problem. My car has now been in local VW dealership since then. They claim no fault can be found. No fault appears on dashboard. Brake eventually works after about third application and 'pumping'. Have been very lucky so far not had a crash.
VW Milton Keynes have been of little help so far. I have NO faith in my car's brakes and it is still under warranty.
Obviously, as this is not an easily replicatable fault it will be difficult to diagnose.
Until I raised a 'stink' the garage expected me to drive it away and seemed totally disinterested in my brakes failing again. I still have their courtesy car three weeks later but they are no nearer to finding the fault.
What can I insist on as the next course of action?

Many thanks.