Hi, Cheers for that, Just took a look and in the manual (Combi 70 V3, Combi 90 V3 combi Max, Vortex Combi 26 and 36) it does indeed show the non return valve.
However i have a supplement manual that covers (Vortex 15/21 Utility and Outdoor Module,Vortex Combi 21 And Outdoor Vortex 21),
Which...