1) In the WB literature, the WB Greenstar Hightflow has a max ouput of
29.2 Kw, and a max flowrate of 20 l/min with a increase in temp of 35
deg C. The 40 HE Plus can output 40Kw and only manage 16 l/min but
has an almost identical efficiency.
Q - How is this possible?

The Greenstar Highflow has an internal heatbank to store energy. It is not
an instantaneous combi, like the 40HE Plus. This is why the Highflow is a
large floor mounted appliance.

2) Are manufacturers boiler statistics accurate?

Yes, but for comparisons, always check the Delta-T. It should be 35C to be
meaningful. Better still, just look at the kW for a condensing instantaneous
combi.

3) Does WB performance degrade after 1 month / 1 year?

No, provided you take precautions against scaling if you are in a hard water
area.

4) I have read that it is illegal to place a pump in an unvented
system. Is this true? I wondering if a flow limited pump exists (and
it legal) that I can put after my mains feed for cold water into
kitchen.

No. It is illegal. For good reason. If you pump mains water, you reduce the
pressure of the incoming mains. You may reduce the pressure below the
ambient, which leads to any leaks in the water company's pipes to draw in
contaminated ground water (very bad), rather than leak fresh water out (only
a bit bad).

This would be able to boost the pressure through the maze of pipes in
my house and ensure that the showers can operate at the maximum flow
available. (Shower has 6body jets at uses about 18 l/min )

If the pressure is not good enough from the mains, then a mains pressure
solution might not be appropriate.

5) Is there such a thing as an unvented Cold Water tank with an
expansion vessle inside? I was thinking that this would enable a short
term higher feed rate of the hot and cold water requirements of the
house.

Yes. It is called an accumulator. It can't fix problems with static
pressure, but it can help with flow rates, at least for short periods. The
following link is to a supplier of such systems. Not a recommendation as
such, I've never used them.

http://www.hwch.co.uk/boosting.html

6) Are there any recent high flow combi boilers equivalent to the WB
Greenstar 440 that are Better that it terms of Continuous unlimited
Flowrate. (Note: I am not looking for a multi boiler info, nor info on
one that involves storing hot water when not in use, just continuous HW
supply)

Then the Highflow is not for you, as it is a storage solution. You won't
find many boilers larger than the 40kW WB, although there are some out
there. However, don't be afraid of storage solutions. You may have had
experience of a wanky 1970s storage cylinder that always goes cold for hours
after a bath. A modern storage solution using rapid recovery coils, or
direct circulation heat banks can maintain the same hot water output as a
combi indefinitely (i.e. it will never run out even if you showered 24 hours
a day). The only difference is that they can boost to many times the flow
rate for a period of time, which is ideal for bath taking. You also don't
have to wait for the genuinely instantaneous combi to fire before hot water
is available.

Christian.