Syphoning action is a pain.
You need an air admitance valve but it must be fitted internally and vertical.
GO TO www.wavin.co.uk
This is the Osma web site
Select
Technical tool box on the top line.
Then Installation guide from the list.
Then select the following in order from each displayed list.
Above Ground Drainage
Waste & Traps
Osma Push-Fit Waste Installation Guide
OsmaVent 40 Air Admittance Valve
This shows diagram and installation instructions
you may be able to obtain the same thing in Holland
This fits 32 & 40mm pipe so you will have to adapt this by inserting a 'strap boss' in the existing pipe work which might be easier than fitting a Tee piece. I assume the existing pipe is plastic as you need to cut a hole for the inner part of the boss to fit in and seal it off with plastic pipe cement [It's similar to the plastic modeling cement]
If you are able to fit one of these then it should stop the syphoning action of the wc and eleminate the drain odour.
If you have problems accessing the site drop me an email via lukesmith8@hotmail.co.uk and I will try sending an attachment of diagram and installation instructions.
Luke
January 2005