Which Worcester Combi is Suitable for me

Hi all
My Glowworm Swiftflow 125e combi boiler is getting the boot at 8 years
old after 5 years of major and minor problems that the maintence techs
from my insurance policy cannot seem to solve. So After doing a bit of
research I have decided to fit a Worcester Greenstar 30CDi for my 3
bed semi with one bathroom.
My choice of the 30 CDI was based on its DHW supply capacity (12.6L/
Min@35c rise) and the power control knob avilaible for it, so that
during the summer I could reduce the power setting. This would allow
the hot water flow to the shower to be kept high with a lower
temperature rise to prevent boiler cycling. Also I do like a good flow
at a high temperature into the kitchen sink.
I have had a few quotes and all the engineers (recommended installers
from the B-W website) never disagreed with my choice of boiler or
suggested an alternative, until yesterday.
This engineer guessed at an 11L/Min flow into the kitchen sink and
suggested a 28i boiler (11.4L/Min@35c rise). He said I would not see
any benefit from the extra power as my water supply did not have
enough flow and (if I understood him correctly) the 30CDi would not
run at maximum efficiency because it would continually modulate the
gas supply. I thought this was the whole point of modern combi design
but is the boiler efficiency reduced when the burner modulates (cars
are very fuel efficent at 56mph but can go faster).
I measured the water flow at the kitchen sink at 12.5L/min cold tap
only and 18.5L/min hot & cold together.
The two boilers have very different heat exchanger designs and the 30
CDi is a much more complex beast than the 28i.
Does any know if there are any reliability issues with the Greenstar
30CDi, is simpler better?
Is the flow to my sink enough to make the most of the 30CDi or would I
be just as well off with the 28i?
any advice would be welcome.
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