Breton

We're thinking of installing the Naviton tubes but will need to erect them away from the house.  Can anybody tell me please if there is a maximum distance between the tubes and the cylinder that ought not to be exceeded?

We are about to install central heating in a building with approx 350 sq m floor area and are undecided whether to go for LPG or Oil; we're not connected to town gas.  Can anyone advise me as to which might be the better option?