I think you need to trust the engineer that probably knows the boiler. or Not.
Assuming you are not talking about the combi, then it has 4 connections 2 x ¾" for the heating and 2 x ¾" for the cylinder.
If it's just 2 pipes going to a zone valve (Y plan) then 28mm + depending on heating requirements.
Put it in writing what the engineer said and hand a copy to the installer, saying you hold them responsible for any addition costs if it doesn't work.
Assuming you are not talking about the combi, then it has 4 connections 2 x ¾" for the heating and 2 x ¾" for the cylinder.
If it's just 2 pipes going to a zone valve (Y plan) then 28mm + depending on heating requirements.
Put it in writing what the engineer said and hand a copy to the installer, saying you hold them responsible for any addition costs if it doesn't work.