Lee Nowell wrote:

Is this valve to prevent some part of the system having too much
pressure?

It does two jobs, one for proper operation, one for safety of an
unvented system.

It says the Outlet pressure preset is 3.5bar

That's the pressure supplied to the H/W cylinder and the cold taps (so
they are equal and mixers work properly) obviously it can be less if
your supply is less

and the Expansion Valve preset is 6 bar. Not sure what this means?

That's the relief pressure that the system will 'blow off' at, and is
the safety part.

Have you got any way of reading the pressure in the system? (e.g a
schrader valve where you would pump up the expansion vessel)

Do you see water dripping through the tundish?