Warmflow 70/90 oil boiler problems

We recently moved into a new house with an oil boiler. It's a Warmflow 70/90 and it has the following issues:

1. H/L Resets - This occurs when the boiler gets too hot I believe. I managed to alleviate this issue somewhat by turning the CH thermostat down to less than 50%. H/L Resets only seem to occur when central heating system is fired up.
   
2. Pressure - Pressure range is between just under half a bar and ~1.5 bars. Is this an acceptable range?
   
3. Variance in water temperature - Sometimes the water just isn't hot enough. The max temperature I've seen from a tap is just over 40C. Last night I turned up the heat store thermostat to max and we had acceptable temperature in the showers, but this morning the H/L Reset tripped again. I've turned the heat store thermostat down a little bit to see if H/L Reset happens again.

I've already sunk £500 into this boiler. The following parts have been replaced:

1. Mixer valve
2. DHW pump
3. Expansion vessel - The old one was leaking.

I've now been quoted £440'ish parts and labour to replace the heat exchanger, but I've lost confidence in this company because we've got no closer to fixing the boiler so far. Will replacing the heat exchanger fix our problems? If not, where do we go from there? Replacing the thermostats?

There's a Warmflow information plate on the expansion vessel which has a date code of 04 02 17. Does this mean the boiler was installed in 2017? It looks much older than 3 years, I wonder if it's 2004? If the boiler is 16 years old, is it worth throwing more money at it or looking at a new boiler?

Ideally the pressure need to be about 1 bar when cold. If the pressure is too low, this can affect circulation and cause high limit problems. A scaled up heat exchanger will give poor hot water supply. How does the flow compare between the hot and cold taps?

Hot water pressure isn't complete garbage. What can cause low pressure when boiler isn't in use?