Ahhh I said this to him, that every pic I had seen with this sort of setup only had a 3 port valve and not the extra 2 port. He said however that the 2 port is a safety device that's needed. So does it meet regs ok if it's not there?
So when you say "wired through the cylinder stats".... forgive me, but what or rather how do I do that?
So as I see it I've got the intergas stat which is a wire with a bit of coat hanger on the end

and the evohome hot water kit that had a small copper temp probe on the end of a black flex. We had presumed both of these could be put together into the stat hole on the tank that the normal white square plastic tank stat would be, and we guessed this tank stat was now redundant. Is that not the case?
Now we've just got to work out how to get the evohome working alongside it. He is on a course/product training day with Intergas today so was hoping to perhaps pick someone's brain. Fingers crossed.
Thanks again Dan for all the help. I know it may seem basic to you guys but to some of us it's all Greek! It difficult to show real appreciation on a forum, it's just words, but I really can't thank you and everyone that's helped me choose such a system enough. The heating and elecs are the spine of the house, so it's so good knowing we have this sorted.
