sounds like theres too much pressure for the ballcock to shut off, change the seating to a high pressure one or adjust stopcock to slow water to ballcock
get rid of the flexi's and get some copper involved, if your not confedent with using copper then use plastic and some push fit fittings, just make sure you get them in full slip and use inserts. that will be alot better than the flexi's.
jesus, we've went from reusing the old ch heating water to using flexi's on the ch system. you will be lucky if i plumber wants to touch it now, i wouldent.