We're currently having a fairly substantial extension built, and will
soon need to decide on heating options. I'd be grateful for advice
and opinions that would put me in a better position to discuss the
options with the builder's "tame" plumber.

First some background on the house and current heating system. We
bought the house about 18 months ago. It's 35 years old, with what I
believe are original (dimplex?) radiators, and the pipes to the ground
floor rads are buried wihin solid floors. The boiler is an Ideal
Classic (of unknown age, suspected 60KBTU), with fully pumped CH/DHW
using a 3 port valve. Hot water cylinder is about 1m high and located
in a large airing cupboard. The loft header tanks are metal and look
well past their use-by date.

I suspect some pumping over into the expansion tank occurs (and may
have done for unknown years) as there seems to be some "gurgling" as
the pump starts and there always seems to be a little air to bleed
from near the pump. The pump itself is set on the fastest of 3
speeds, as it fails to cold start on any other speed.

Currently we only have one bathroom, and four sequential (pumped)
showers make the last person in the queue shiver as the hot water runs
out.

I've basically avoided doing anything with the system as this
extension and revamp has been "around the corner" for ages now. The
extension will add a kitchen (6.5 x3.5m), small utility room (2 x
1.5m), bedroom (5 x 3.5m) and en-suite bathroom (1.5 x 3.5m). I've
done a very unscientific flow test and measured about 15l/min at the
kitchen tap.

What we're ideally looking for is a system that is capable of:
1. heating the larger house
2. coping with at least 4 showers (if stored DHW)
3. hopefully delivering 2 decent showers in parallel (and by decent I
mean more than at header tank pressure)

The first choice would seem to be whether to keep the existing boiler,
or replace it for greater capacity / reliability / efficiency.

The second choice would seem to be whether to start from scratch with
the radiators and buried-in-the-floor pipework.

After that, options so far include:

1. Keep system pretty much as-is, but install larger and faster
recovery indirect cylinder, CW tanks, etc. Add pump for showers.

2. Pressurised cylinder. I've some concern about running existing
radiators and pipes at higher pressures than they're "used to"
(especially a leak in a solid floor would not be fun)

3. Largeish combi. Concern about water pressure as above, also I've
some doubts about ability to deliver 2 showers in parallel.

4. Thermal store / heat bank. Seems ideal in that it would provide
header tank pressure CH, with mains pressure DHW. Not sure what the
drawbacks might be other than maybe cost or possible lack of awareness
by plumber.

Any comments on the above (or any additional) options welcome, along
with any other suggestions. Recommendations on makes / models to
consider would also be gratefully appreciated.

Thanks in advance,
Martin