Basically what is required is to measure the flow coming out of the boiler to see if there is some fault/restriction in the boiler so simply put, disconnect the outlet from the boiler, turn the temperature control to minimum, (despite what I said you do I think need the boiler diverter valve to DHW) and let the water flow into a bucket for a timed period of one minute and just measure this with anything handy like a one litre jug.