Central heating header tank over flow
Let me say first that I have never worked with this type of boiler, but the question you ask is similar to a few problems I have had with boiler systems here in the USA. I do not know if this is the same as your installation but I will tell you what I think may be the problem.

Someone specifies a larger pump than necessary or a larger replacement pump for an existing installation. The system is filled with water, vented, and put into operation. When the pump starts, the water pressure on the inlet side of the pump falls and the PRV (heating system pressure regulating valve) which is installed on the piping near the inlet side of the pump senses the drop in pressure and adds water to the system to correct for the reduced pressure.

When the boiler and the pump shut down, that extra head the larger pump exerted on the supply piping is the felt on the whole system. When this happens, the relief valve (which you call the overflow tank pipe) opens to get rid of the excess water that was introduced when the pump started. That extra water raises the whole system pressure above the set point of the "relief valve" when the system shuts down.

I do not know if this is what is causing your problem but it sounds similar. You may need to go back to the old pump size to correct the problem. Changing the pump just because a radiator was added or changed is a foolish mistake done by the uneducated in their trade.