Michael Chare brought next idea :
If you are saying that the gas meter has been changed could it be that the
gas supply to the boiler is now restricted? Is the main valve turned on
properly?

Easy to check, if there are other gas appliances...

When the boiler is running flat out, turn another appliance on and see
if its flame height varies when the boiler is and isn't in operation.
If it does, there is a flow restriction.