Girl friends car, is under 12 month RAC parts warranty, The car has a issue with what vcds points to the lambda sensor, how ever when i take it too the garage the test it with snap on scanner and it doesn't show anything...

snap on diagnostics are indeed rubbish. usually miles behind other equipment and nowhere near as in depth.

Garages who don't normally get involved with diag work end up with a snap on tool in the hope that they can trust the brand but in reality there is far better equipment out there.

How do i go about saying/ proving this? Should i take my laptop to the garage and scan it for them? they can then pay ME £80 ?

My brother has one and for getting a bit more than the basics about any car you come across, it's alright. It can't really do anything on a VAG that I can't do with VCDS Mobile on my phone though.

What do skoda/ vw indies use?

vcds, vas equipment, autologic, bosch kts or all of them.