Warmflow 70/90 Oil Fired Combi rads not getting hot?

I have a Warmflow 70/90 Oil fired combi and the press ure relief blew the other day, i checked the expansion vessel which was faulty so i fitted a new one and replaced relief valve, the boiler fires and runs ok but rads are not getting hot, after 5 mins or so boiler cuts out , all rad valves are open and system has been flushed, system has been bleed and pump is working, return pipe not getting warm at all, but bypass pipe is,


any ideas?