Background:
We live in a 3 bedroom house-on-a-hill. From the front it looks like a
bungalow. Once inside you go downstairs to the bedrooms. The (non
combi) boiler is in the garage on the front. The hot water tank is
downstairs in the airing cupboard (pumping hot water downstairs against
convection, hmm who thought of that one!). There is a header tank in
the loft. The CH radiators are all on Microbore pipe. In the 6 months
we have lived there the CH top up tank has required about 2-4 litres of
water to top it up.
We had a combi boiler fitted in our last house (non condensing £1000).
Running a bath took long enough to undress our toddler and clean his
teeth and get him ready for bed, just about acceptable, certainly not
quick. Showers were great with a very good flow and pressure, enough to
make you want to stay in there longer.

Situation:
We want to extend into the loft and so need to lose the header tank.
We intend to have a bedroom, nursery and shower room in the loft as well
as a sizable landing (there's plenty of space due to the layout of the
house).

I've been told that microbore pipes and combi boilers don't mix.
Is this true?
I've also been told that if your system leaks a combi boiler will make
it worse because of the pressure. (Remember its needed toping up already!)
I would imagine the plumber who fits the new radiators in the loft may
want to avoid using microbore and so we would have both standard and
microbore radiators. Is this an issue?
I have no idea how much it would cost to replace all the existing CH
pipes (floor boards not sheets, at least in some rooms, no concrete
floors afaict) and radiators (3 bedrooms, kitchen, dining, lounge, hall,
lower floor hall, bathroom and washroom (10 rad in total) and whether
this would be a better solution than using the existing microbore.
There will be a plumber coming in to add new rad.s to the loft anyway.
Existing CH and boiler is original fitted in 1992 when house was built.

Things never seem to be simple though.

ATM my understanding is our DHW comes from the hot water tank in the
airing cupboard downstairs (there's also a pressure vessel in there too,
not sure what for though).

Siting the new boiler in the place of the old bolier in the garage puts
it just the other side of the garage wall to the kitchen sink. The
exiting bathroom is directly below that and the new shower room will be
just above and to the back of the house so the pipe runs to the majority
of the taps from the boiler will be short. (Our main gripe with the
combi boiler in the old house was that it took ages to get any hot water
through and then if you were just rinsing the pots you had to leave the
tap running at a fair lick in order for the water not to go cold which
it would do if you turned the tap off.)
The current hot water pipes from the boiler go to the airing cupboard
and the DHW pipes come back from there. It seems silly to send the
water half way round the house so we are intending drilling through the
garage wall into the space where the washing machine currently is in the
kitchen and connecting the DHW feed from the new boiler to the end of
those pipes. I'm presuming all the DHW taps are on branches off one run
from the exisitng hot water tank so capping of the other end at the hot
water tank (with a drain plug) will mean we still get hot water to the
all the taps.

For the CH we were going to try and locate the existing loop and break
into it as close to the new boiler as possible and close the loop in the
airing cupboard where the hw tank is.

My questions a

Does the above sound feasible? (IE are there any glaringly obvious
mistakes I've made or things I've overlooked.?)

Are there issues with microbore pipes for CH?

Can you mix micro bore and standard piping in one CH system?

Considering the minimum job for the loft will be addition of 4
radiators, new boiler and the plumbing to (re)move the HW tank from
downstairs would replacing our existing 10 radiators on micro bore with
new radiators on standard pipe add significantly to the price (+10%?,
+50%?, double? triple?)???

Where is a good place to start looking on the net for prices for
boilers/rads etc.

We will then have the possibility of using 2 showers at the same time
(one a floor up from the boiler and one a floor down from the boiler if
than makes any difference). I'm presuming some sort of HW storage tank
(as opposed to a combi) will be required to allow both to be used
together. Is this the case (most "Combi yes/no?" threads so far have
only concerned one shower being used. Could a combi and mini tank be
used or would the best idea be to go for a mains pressure storage tank
solution?
As the back half of our garage has been converted to a store room there
is not a space issue with putting a hot water tank in there with the
boiler but I'd prefer it to be compact as possible.

If I was to redo the exising pipes and radiators myself what's the best
way of getting the pipes laid when their direction is across the joists?
(I want to minimise pulling up the floor. My intention would be to
use copper pipes from the radiators into the under floor space and
plastic pipes to link these as I've been lead to believe that the
plastic pipes are easier to work with.

Any comments appreciated.

Chris.

PS Apologies for the long post (as the bishop said to the actress)