Anyone used one of these for testing their skoda for head gasket leak. With the expansion bottle having such a large filler cap not sure if the tester will seal around the neck of the bottle.

 

http://www.ebay.co.uk/itm/COMBUSTION-LEAK-TESTER-KIT-PETROL-DIESEL-HEAD-GASKET-BLOCK-FX-53-100ml-FLUID-/272454398909?hash=item3f6f8c33bd:g:pxUAAOSw4GVYJMhP

I've used them on other cars but not a Vag with the large opening.

I don't think it's entirely critical for it to seal around the tank unless you have an exhaust leak in the engine bay or wind blowing the exhaust towards you.

You should be able to get a compression tester for around the same price which would limit any false readings.