new radiators

[godfrey1971]

I have to add three more radiators to a system that a builder has part installed he connected new rads in the upstairs extension and did the first fix to the downstairs extension the system is fed from a combi boiler (new) , I know the down stairs pipes are capped but the only drain cock I can find is under the wall mounted boiler in the new utility room,cant find any on the old radiators either, so i'm in for fun and games when I cut the caps off, any tips or help when filling system would be grateful . the builder has been giving my relative the run around and so I dont think he is coming to finish the work,the heating system is working at present ,
thanks