Heatloss calculations are an important part of sizing a boiler requirement.
An amount is added to allow the requirement for hot water production .. However, it is important to take into account the construction of the property. This varies the heat-LOSS and therefore the heat REQUIREMENT significantly.
To do proper heatloss calculations means doing as I stated before.
The CDi is larger in heat output and slightly better efficiency, as it condenses in Hot Water mode too .. I have taken a look at both manuals ..
and the brochure ..
http://www.worcester-...s-boiler-brochure.pdf
The Si is 24Kw, the CDi 30 or 40.
We can't help much more without doing calculations with you providing every room measurement and the house construction details.
You could ask 'them' if they are doing heat-loss calculations .. or just guessing (which they must be) and ask another company to quote on a Heatloss calculated system.