How do I force pump to force supply to radiators at top of house,
If you are not getting heat to the radiators in your loft area, the problem is probably not enough system pressure. GO to the loft radiator and open the vent. you should get water out of that vent or air first then water. If you do not get water out of that vent, you do not have enough system pressure. Go to the boiler and check the system pressure. If the system pressure is 10 psi then the water in the system will be 23.1 feet high. (1 psi = 2.31 feet high). You could measure from the top of the highest radiator to the pressure gauge and divide the distance in feet by 2.31 and get the required system pressure, and add 2-3 psig for safety. From your writing, I will guess that you have what is called a monoflo  heating system. If that is the case then poor installation practices may cause one radiator to heat quicker than the others. By the way, where are you located since you are using mm instead of inches as we use in the USA. (0.0999Kg/CM2 = 1 meter) You should not have to close off 1 radiator to get heat somewhere else. All radiators should heat at about the same time.