You will be able to use the exsisting boiler and add a zone valve for the heating circuit, zone valve for the hot water circuit & you may want to control the new extension with a zone valve & thermostat.
(assuming the boiler is sized correctly)
You could use the DHW side of the combi boiler to...
In order to be exempt from building regs consent, the conservatory must:
Have at least half the area of the walls formed as windows, doesn`t sound like that will be the case.
If you decide to put a radiator in there, you should tee off the primaries of the boiler, and have the extension on...
Is it the duo-tec/platinum 40he?
The output for the central heating side wont be as high as 40kw, but from memory the burners on these are fully modulatin according to the heat demand required.
Your installer should be able to set them maximum heating load within the parameters of the boiler also.
:D Now you are just being pedantic......
The statement " so putting in a larger boiler does give you a better flow rate" Was in refering to the example i had used above of incoming mains 20l/min dynamic...
It goes without saying, I would assume any gas installer worth his salts will measure...