To be honest, where to begin? Is the back boiler a proper heating boiler for rads, or is it just for hot water? Small back boiler no good for rads. If it is for rads, then why was it only joined to hot water?( A large back boiler would have been too powerful.) It might be best considering a new boiler, even multi fuel stove? Assuming it is for rads then normally two separate pipes, 22 mm usually, come from the other two tappings on the fire, to connect to main pipes for rads. A pump and check valve first. Never fit any valves on gravity pipes to cylinder. A check valve at oil boiler also. This system is probably easiest option I am guessing. Motorised valves could cause problem as to how to heat both circuits from fire. It can also be piped slightly differently with an injector tee. Other options are types of neutralisers. Why not look up Link up systems for solid fuel to see pipework arrangement? I think only experienced engineers should do any of these.
