Andy , may I suggest that you bit the bullet & get professional help /advice from a local to you qualified plumber who plays with such systems .
Some 12 yrs or so ago acquaintances of ours had their back boiler blow as the " expert " didn't have put a vent in the system. When the boiler exploded it showered their lounge with boiling water & bit's of plaster etc when " friends " sorted out their heating set up for them , luckily no one was in the room at the time . The whole way things were set up left a lot to be desired wrt safety & efficiency as far as I could see.