Is there meant to be an O ring on the cap of the expansion bottle????

Which expansion bottle?

Which expansion bottle?

There is only one, for the engine coolant

If all the Superb MkII vehicles have the same expansion tank there is no o ring, the cap has a definate click lock and no provision for a ring. There is though an o ring on the pressure relief unit in the centre of the cap, I did find on My second 170 ps MkII that removing the filler cap a week after a 400 mile run that there was a vacuum in the system, the centre unit was quite tight so I lubricated the o ring, the centre then moved easily and when removing the filler cap when cold after long runs no vacuum was still in the system, the first thing I did then with my next two 170ps MkII vehicles was to lubricate this o ring.