Thanks Christian. I am actually going to change my boiler at the same time
as I do my shower + bath rooms. I currently have a normal convential
boiler
gravity system with hot and cold water tanks. Would it be worth changing
the
boiler to a combi that gives mains pressure, are these less efficient than
condensing boilers?

The combiness of a boiler is independent of the condensing nature. A
condensing boiler is more efficient than a non-condensing one as it exhausts
the flue gas at a much lower temperature. They are much more efficient even
if not run in condensing mode, so you don't need to worry about your
radiator sizes.

A combi boiler is normally more efficient than a storage based hot water
system, as there is no large store of hot water to cool off. The main
problem with combis are that they have a limited flow rate. However, this
flow is done at high pressure, making them excellent for showers.
Unfortunately, bath filling performance can be poor. Some "combis" aren't
really combis at all, but are integrated storage heatbank-boilers. These are
better than non-storage combis for baths, although the store is soon
exhausted.

The other alternative is to install a proper mains pressure hot water
system, such as a heat bank or unvented cylinder. This option will provide
the best solution, but will be considerably more expensive and take up much
more space, if you don't have a convenient loft space or suchlike to use up.

Any mains pressure solution depends on having sufficient static pressure and
maximum flow rate from your mains supply.

Would the pressure be significantly reduced in the shower(below the power
of a gravity shower) if someone filled up the bath at the same time as the
downstairs shower was going?

That would depend on the pump. A 1 bar would probably drop flow rate.
However, a 3 bar pump with large bore pipework might not be badly affected
at all, especially as the shower is downstairs. If it does become a problem,
choking off the bath taps (with their isolator valves) a little should
restore a bit of dynamic pressure for the shower.

Christian.