Hi I spend all day every week repairing baxi combi boilers, the Halstead is ok but parts can be expensive' you can normally get 4 years before anything go's wrong with this. Wickes do their version of this boiler for £475 including flu. The best is Valliant and next to that is worcester 230...
Hi you can do this a few ways, check the manifolds you should have 1 up and 1 down to see if they have spare connections if so just connect to them, 1 pipe to the flow and the other to the return manifold and repeat for the other radiator. If no spare then you can tee into the feed flow and...
Auto is usually located inside boiler near relief valve and usually has a car tyre like cap on it.
When filling system it is best to have all bleed screws closed. open filling loop a tiny bit starting on the ground floor bleed each one at a time until rad is half full then go do same up...
Its sounds like you have a lot of dirt in the cold storage tank, check by looking into the tank, if any, turn off stop cock to this tank clean tank out the best you can, eg remove all floating objects then open cold bath tap fully until tank is empty then close tap and refill storage tank as it...
A combi can cope with hot water and CH in the average household, it wouldn't cope in a busy bed & breakfast with more than 2 bathrooms/showers running at the same time. Then you would need 2 combi, each running its own independent part of the system. I would go for a combi any day