I have completed the connection of Geberit to MVHR, so here is the detail FYI.
There seemed to be interest in the JSH and PeterW versions.
Plasterboarding and second fix is delayed, so the fans are not yet functioning, so the ventilation performance is as yet unconfirmed.
Geberit model #111.353.00.5 has a Y-branch for extraction of air[smells].
I removed the translucent plastic duct [part of the Geberit] attached to the Y, cut off and Siga taped up the end that linked to the top of the water tank.
Into the Y was inserted an adapter from 50mm Euro to 1.5" UK [RHS: the most expensive piece of kit in the house @ £12 for a short piece of tube that was 50mm ext and 43mm int diam], followed by two UK reducers. This led via various slow bends through the house in 32mm waste pipe to near the MVHR unit, where it was connected into the standard radial ventilation pipe.
32mm waste pipe was used as it fitted within the service void and was generally unobtrusive and easy to slip in. I did not see that a large diameter pipe was necessary, nor would it have been easy to fit another 75mm pipe in.
http://www.sanifix.co.uk/conversion_fittings.html
Connection to the ventilation pipe was via 50-43mm and 43-32mm reducers [white] and a 50-82mm reducer [grey], which fitted neatly into the ventilation pipe with a rubber sealer ring.
Any questions please ask.