I read that installers have a habit of vastly over sizing boilers.
I doubt it. Installer has very little to gain from oversizing the boiler. The difference in price between say a 24 or 28 kw is about £50.00. You are perhaps confusing a system boiler output against a combi boiler output which is like comparing oranges with apples.
What is quite common is that an installer may ‘assume’ gas line adequate when it is not and the finish the job, if he indeed is half way bothered, to fudge the figures in the benchmark which more often than not is a blank sheet