Aqualisa Aquavalve 600 thermostatic?

I have just replaced the White body unit and the cartridge of this shower. (Actually the old unit was a Gainsborough Ambassador but the parent co' Aqualisa Aqauvalve is identical.. My problem is the 22mm outlet at the top of the white body leaks around the thread of the screw "nut" which secures the 22mm copper pipe into the body. When removing the old body there was a ring seal around the copper pipe plus the "nut" which remained and I used again when securing the new unit. I see on aqualisa's website cutaway diagram of this model it indicates this item number 11, "White outlet assembly kit" but the diagram is too small to see and in anycase does not indicate how the copper pipe is sealed into that top outlet port of the "White body" by this "Assembly kit"
Anyone any idea how this connection is rendered water tight?