It appears that the 12mm pipework and fittings is actually 1/2″ and is more expensive than the 15mm parts. I will be using John Guest (JG) speedfit parts only from now on.
I am not far from Magnum Motorhomes so will take a trip up there as soon as funds allow.
The other reason I am moving to 15mm is that the shureflo pump recommends not less than 1/2″ internal diameter of the pipe which is 12mm internal not 12mm external. The 15mm pipe has a 12mm internal diameter.
I am hoping that next week I can finally get this project on the move and do more than minor messing about jobs. Some serious progress is needed…..