spec me a new system!

[colddraughty]

i am renovating an old property, and have come to the point of needing a heating system. i have fixed the roof, insulated it, and replaced 21 draughty windows with new double glazed ones.
currently the house has 2 combi boilers fitted, one serving hot water downstairs and heating upstairs, the other vice-versa. one boiler is a very old ferroli, and the other a new ish vaillant. i am resigned to the fact i will be losing them both.
the house has 4 bedrooms and 3 bathrooms. i am taking one bathroom out, and making another into an en-suite, meaning the property will have 1 bathroom and an en-suite.
the ceilings are quite high - about 10ft, and the rooms are quite big. i filled out a radiator calculation form, and it reckoned that the four largest rooms would need about 11,000 btu each.
so, i have an opportunity to spec pretty much whatever i want, but the question is, what? i know nothing about the subject, and am getting conflicting advice. BG reckon an unvented system is the way to go, as after turning 3 taps on, we measured a flow rate of 21 litres per minute, about 7l/min for each tap. BG think this means we cant have a pressurised system, but are returning with a surveyor to check properly.
another fitting company reckon this is no problem, and a vaillant pressurised system is the way forward.
so, any ideas / suggestions / tips / things to avoid?

 

[mickyg]

whats up with the combis? and why can't you keep them?

 

[colddraughty]

not much wrong with them i suppose. the ferroli is temperamental and very noisy. the vaillant seems ok.
the whole system is a little odd, there is a thermostat halfway up the stairs, so when one boiler warms downstairs, the warm air reaches the thermostat and turns off the upstairs boiler.
also, they are located quite a distance from the en-suite, meaning there is much running of water to be done before anything warm appears.
the ferroli is quite old, and i think if we were keeping the combis it would be replaced anyway, but as i say the vaillant seems fine.
we are replacing the bathroom fittings, and are hoping to have well spec'd bathrooms, and one of our wants is a nice powerful shower in each. not sure if the combis can manage that between them, or they certainly aren't at the moment!
in short, if i was on a budget, i would replace the ferroli and move on, but i would rather take the opportunity to get the system right. after this is done, there are a number of other disruptive jobs to be done, such as fireplaces, and flooring followed by some light wall moving then a new kitchen and lots of decorating. i don't want to get to the end of that lot, then be unhappy at the shower / heating performance.
if i can get it right with the combis, shout up - open to all suggestions!

 

[mickyg]

If you can get the right flow rate then you could use an unvented cylinder. This could be heated by the vaillant combi you already have, which could also supply a utility room or something similar via hot water supply.
What vaillant combi is it?