Radiator cold at bottom after powerflush & new pump

Had no problems with heating, in fact it was always very fast heating up and nice hot radiators, no cold spots. However, due to the colour of the water BG said during annual service that I needed a powerflush. Now had the powerflush and engineer replaced pump & motorised valve at same time. But immediately I notice the largest radiator (double) in sitting room which was always fine, is now only hot at the top and cold at the bottom. Also, I'm now getting some faint kettling . Would this be due to the new type of pump (a blue BG Multihead G model), old was a red coloured one. I notice a big difference in pump noise, new one is very quiet, old one was much noiser in operation. Which makes me think old one was more powerful. (?)

Old one wouldn't have been mmore powerful. WHat speed is the new one on?
Radiators DO start off hot at the top and cooler at the bottom, and stay that way if the flow through them is insufficient. Your problem, I mean their problem, is almost cetainly one of Balancing the rads. It could for instance be that most of the flow is going through the radiator nearest the pump. (do a search on Balancing)
Haul them back!