We had a new central heating system installed just over a year ago
but the tempature of the hot water seems to be variable at different
times of the day and in different parts of the house.

You might find that when the temperature is cold, it is because the mains
pressure is high and promoting a higher flow rate. Next time the water only
gets tepid, take a container of a known size and time how long it takes to
fill. Post back with the calculated flow rate.

Your combi boiler is capable rising water temperature by 35C at 11.4lpm.
This time of year, your mains are probably around 5C. So, if you get around
11 lpm, the water temperature should be 40C, which is warm, but not exactly
hot. If you expect water at 55C, this is an extra 15C rise, which will drop
the flow rate to around 8 litres per minute. If you draw more water, the
temperature drops. There is nothing that can be done to improve this, except
by replacing the boiler with a much larger model, or installing a hot water
storage system.

As for the pressure gauge, the water in the system will expand considerably
when hot. This would cause a rapid pressure rise, but is absorbed by the
expansion vessel instead. This is usually sized to limit the pressure rise
to around 0.5 bar or less. When cold, the system should be pressurised to 1
bar. So, next time the system is cold, attach the filling loop hose and
increase the pressure to 1 bar. Don't forget to disconnect the hose
afterwards.

Christian.