1. How many baths and showers do you have?

1 bath, 3 showers

2. How many people live in the house?

5
3. Does anyone prefer baths to showers?

Yes, I prefer a bath, the wife, a shower. The kids prefer neither ;-)

Well, you definitely wouldn't get away with a non storage combi with that
lot.

4. What is the maximum flow rate at your kitchen sink? (use a bucket
and stopwatch to measure).

150 galls/hr

I'm worried that this is a bit low. I make this 11.4lpm. This basically
isn't enough for a decent mains pressure system. However, it may just be
that you have a tap that can't pass more water than that (many modern taps
have narrow passages and are flow rate limited) and that the supply isn't
actually that bad. You'll certainly want to investigate further to ensure
that the real potential flow rate is higher. If you can get at least 20lpm,
then mains pressure will be OK. 40lpm and it will be good.

You may need to turn off your stopcock, and remove a section of pipe and
measure flow rate from there to get the real story. Although a bit drastic,
it is better than spending thousands of pounds on a system that turns out to
be inappropriate.

5. How happy were you with your old system when it worked? (I
presume it was probably a gravity hot water cylinder fed by gravity
circulated water from the back boiler).

I wasn't. Cold rads, and it took a long time for hot water to reach
the kitchen.

If you intend to install a new boiler in the kitchen, and the mains pressure
turns out to be OK after all, then I'd recommend installing the heat
bank/unvented cylinder where it is now, presumably near the bathroom. Then
install a combi boiler in the kitchen and run just the kitchen hot tap from
there. As there won't be loads of pipework between the source and the tap,
the hot water should come quicker. A combi version of a boiler is usually
only a few quid more than the system equivalent, so there would be little
additional cost.

If the flow rate really is bad, I'd recommend replacing the boiler with a
modern condensing system type. Replace the old cylinder with a modern Part
L, and convert to fully pumped Honeywell S plan. Fit a quality brass bodied
heavy duty shower pump (such as Stuart Turner) off a flange at the cylinder
and feed this to all the showers and the bath. Other outlets should use the
unpumped hot to avoid running a noisy pump in the early hours when washing
hands (or clothes). This may require additional pipework.

Christian.