Why would the input gas pressure fall when the central heating boiler calls for heat?
When the boiler is not heating, no gas is flowing, and you will measure the static input pressure of the gas supply line. The input pressure at the boiler drops when the boiler calls for heat because the gas is flowing during heating, and pressure drops occur in the gas piping upstream from your boiler due to the gas flow. 

Without knowing the flowing gas input pressure before the annual service, it's not possible to say whether the present situation is abnormal or the service is the cause of less fierce flame. But an increased input pressure drop compared to before could be caused by increased obstruction in the upstream piping (flexible gas pipe being partially crushed, corrosion or debris and narrowing of rigid pipe, etc.) or decreased gas flow resistance in the boiler after servicing (removal of build-up from gas orifices and plenum in the boiler?). Another cause of could be a new gas leak in the boiler after the gas valve, however I would think you would smell gas if this were the case.