Boiler sizing

[bigdom]

I've had the first heating engineer round to quote for replacement boiler, keeping the current low pressure system set up. Boiler quoted, appears larger than required according to online sizing calculations, although looking at the boiler specs,they all start around 7kw then go up to different outputs, all appear to be roughly the same efficiency, so would it be good to have some back up power in reserve?

[bigdom]

We have an extension going on the rear, and the current boiler wouldn't cope with the extra.