No heat in downstairs radiators

Your plumber has probably opened all the lock- shield (return) valves to enable the cleaner to circulate around the system more freely,thus unbalancing the system ,when he comes back he will choke them back down and the heating should be balanced again.i would have thought that he is trying to avoid a power flush ,because on an older system it can do more harm than good.

Check there is no valves closed stopping the circulation of water , als if it wasn’t working after the install go back to that engineer as it was their responsibility to check it was working before leaving

Check the lock shields are open by turning them anti-clockwise. Make sure all thermostatic radiators valves are turned to 5. You may need to get someone in to balance the radiators

If the heating was working both up and downstairs before then there’s no reason why it’s not working now. Isolate all the radiators but one which is not working, turn the heating on and see what happens. You should have some circulation. Ensure both valves are open. If not you need to call the boiler installer back. When he installed the boiler he should have cleaned the system it’s part of the regulations. If the system was not cleaned then you may have a blockage. One last thing if your system has been converted from conventional to a combination he may have crossed a pipe stopping circulating downstairs. Hope this helps.