Reimo do the german spec gas fittings - I think you need an Überwurfmutter, Ø 8 mm.
look here:
http://www.reimo.com/en/75117-ueberwurfmutter_ml_8/" onclick="window.open(this.href);return false;
Do a 'reimo uk' search to find the best place to get it from. Concept Multi car are very good, but...