I doubt that anyone else will have the same problems with gas/air leaks when using these, I also had problems getting proper readings from my track pump pressure gauge due to poor sealing. Anyway, in the unlikely event that anyone else has the same problems I have found the answer, read the manual :-( The installation instructions are included below, I didn't have them when I got my wheels.
VALVE EXTENSION INSTALLATION
Note: Install valve extension BEFORE mounting tire.
1. Unscrew tire valve locknut as far as possible, and tighten in the fully open position.
2. Stretch a wrap of Teflon plumber’s tape (available at a hardware store) around the end of the valve so that the tape extends over both the narrow valve cap threads and the wider valve body. Make sure that the tape does not stretch over the air opening at the end of the valve. This provides an air seal between the valve and extension. Plumber’s tape comes in several thicknesses. If thinner tape is used, more than one layer might be needed to seal properly. Tape should be replaced each time the extension is removed.
3. Screw the valve extension onto the end of the valve and tighten, using a 3mm Allen key inserted into the end of the extension.
4. Mount tire as usual and inflate with a “push-on” type pump head. Check for leaks. If a CO2 inflator is to be used with the extension, it must have a “push-on” type head. Always check valve, pump, and inflator compatibility before riding.
5. Air can be released from the tire by inserting a piece of wire (a section of spoke works well) into the end of the extension to depress the valve nut.