To clarify i am talking about two types of installation, the correct way and the fruit loop way, i have only seen the correct way in books(i am not a plumber just a gas fitter), but i have seen the fruit loop way a few times and any posting about not being able to add the a back boiler/stove would be aimed at the fruit loops, you and Villa1 may not be aware of how many fruit loops are actually out there, i have seen thousands of installations and the fruit loops seem to out number competent installer 10/1, so i wouldn't be as confident as Villa in finding someone with the skill to install safely, Gary.
I have been offline for a few days and am surprised to see so many posts. It will take me a while to read through them! Thanks for all the comments/advice.