'Big yellow thing' is your Hot Water Cylinder. Boiler will heat the water in the heating system, and the pump circulates it around, the 3 Way Valve then opens according to heat demand, and allows flow of hot water to either the Cylinder, Radiators or both.
If the upstairs Radiators are getting nice and hot, then it is a circulation issue with the Radiator pipework, nothing to do with the 3 way Valve, or anything else for that matter. Microbore is renowned for blocking, it either needs a proper clean out to allow full flow, or replacing with 15mm pipework.
What sort of Flush has been done? I doubt anything less than a Powerflush with a suitable machine will unblock the microbore.
If the upstairs Radiators are getting nice and hot, then it is a circulation issue with the Radiator pipework, nothing to do with the 3 way Valve, or anything else for that matter. Microbore is renowned for blocking, it either needs a proper clean out to allow full flow, or replacing with 15mm pipework.
What sort of Flush has been done? I doubt anything less than a Powerflush with a suitable machine will unblock the microbore.
