For me one frustration was to be told that there was inadequate water flow but there being no means of measuring what flow actually was. ie how far off the required 1600 litres per hour we were. Also, there seems to be no scientific way of assessing the pipework and pumping requirements except by trying things out.
Actually there is a method; it's called "calculating the index circuit".
This works out the pressure loss in the system due to pipe sizes, number of bends, size of rads, system layout etc. This info, together with the required flow rate,enables the engineer to find, from the pump manufacturer's catalogue, the correct sized pump.
The problem is that the procedure is time consuming, so installers don't bother. You also need to know exactly where the pipes run etc, which is OK for a new system where the pipes are installed from scratch, but on an existing system the pipes can be buried behind walls and under floors.
In addition, most installers can guess what size pump is required, based on past experience.
The problem, in your case, was caused by two things: an oversized boiler and too small a pump.
You mentioned earlier that you have a large 4 bed house with solid walls, loft insulation and some double glazing. My last house was a five bed Edwardian semi over three floors. Solid walls, loft insulation and the majority of windows were the original single glazed wood framed. The boiler was a 24kW floor standing cast iron Potterton Kingfisher. The house was always nice and warm.