Could a leaking boiler be fixed

You have probably fixed the problem - at least temporarily.

The boiler requires that this condensation pipe is clear and unobstructed and removing the ice from the end has cleared the blockage. However, it's likely that the pipe will freeze up again - especially in this cold weather.

The solution is simple - insulate the pipe. Either use something like this - the pre slit stuff will make it easier to fit as the pipe will be clamped in a number of places. If you can't get hold anything that's the right diameter for your pipe at short notice, wrap the pipe in bubble wrap. That should be enough to keep it from freezing.

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.