apologies for multiple post last time - I was getting the message to
try again and again ... from google

3) heating on demand is more economical from storing water
Is it though? If you have to run a big ****-off boiler flat out to heat
the water, is it as economical as a modulating boiler running at tickover?
ok - to heat up X litres of water from temp A to temp B you need
certain amount of calories from gas. As far as I understand it, it
doesn't matter if you heat it
fast (combi) or slow (storage tank) - you still have to burn the same
amount of gas. Big ****-off boiler will run flat out for say 15mins
while you take shower. At the same time modulating boiler will
tickover for 1 hour in reality burning the same amount of gas. Now,
when you store the water in the tank heat will escape and you have to
burn some more gas to keep water at temp B all the time. This is where
combi wins I think (at least in terms of energy savings)
Additionally if you work irregular hours you don't have the trouble of
programming when exactly you need to have your hot water.

4) I can free some loft space for future conversion
do I miss some important point here ? feel free to comment

The size of a smaller boiler and small (150L tank is not much greater
than a 20KW boiler.
Neither is the cost a great deal different in my case. YMMV.
Yep - I agree. But you also have to have run additional pump (press
tank) which presumably is noisy and cost you electricity.

it's only two of us in the property, so what I am really after is a decent flow from a shower when someone is doing the dishes.
You won't get that from a combi.
hmm... - no good, will have to discuss that with my girlfriend then =(

I stayed in a house once..with someone with a teenage daughter. The HOWLS of anguish when I turned on a hot tap
to wash my hands from the teenage daughter showering..
how about using thermostatic mixer showers ? Flow would drop but you
won't get temp fluctuations ... Just like you have with your tank
setup.

Seriously look closely at what peak hot water rates you CAN get with a combi.
15.2 ltr/min for big ****-off Vaillant Ecotec 837. Initially I thought
this would be enough for shower + kitchen tap (i.e. dishes).

Mmm. The pressurized tank will go anywhere..doesn't have to be in the
loft. Could be in the garage..
As could the boiler.. Mines in a corner of the loft as it happens..

Yes I am sure I could find some corner space even with a loft
converted. Thanks for your input I will definitely consider this
option.

Mike