thanks for your reply!

In discussion with the plumber, it turned out that taking the old, but
perfectly serviceable boiler and coupling it to a mains pressure tank
and converting the rads to sealed Y plan would be the cheapest option of
all.

I take your point here. However my current system is really old and
badly
designed. It's open vented system with hot water cylinder and 15+
years old
baxi boiler. I say badly designed bacause I think I have undersized
radiators
and pipes (8mm copper). All I can get during winter is less than 20
degrees air temp.

Not wishing to put you off, just double check that what you are doing IS
the best cost-benefit..

cost benefit is the main reason - here is why I think new combi is
good idea:

1) my current boiler is old, I had few problems with it this winter
and would like to
avoid the situation where I have to replace it quickly in the worst
moment (next winter)
doing rads/piping/controls myself I can save some money. Remeber
it's the labour
cost that's the most expensive in CH
2) new boiler will be condensing model, which means savings on gas
bill
3) heating on demand is more economical from storing water
4) I can free some loft space for future conversion

do I miss some important point here ? feel free to comment

I certainly have never met a combi yet that can match my peak hot water
flows here, with a pressurized tank.
I agree. However 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.

Or can have an immersion heater if you run out of oil or gas..
As I mentioned - I plan to convert loft at some point and have a
second bathroom
with electric shower as backup. As we shower quite a lot and hot water
cylinder is a
bit to small so we use electric shower at the moment.

Nor yet one that delivers full pressure hot water even when the power
goes out..well, for a while, anyway :-)
can't argue with that =)

again thanks for your input,

Mike