Problems with Ravenheat Combi RSF Fanned?

Boiler has no markings on, but have manual that says...

Fanned Combi

RSF 84E - RSF 100E
RSF 84ET - RSF 100ET

We have a 5 year old ravenheat combi boiler, always had intermittent problems, but they have always sorted themselves out, but there is a new problem & it isnt going away, help!

central heating works fine, but hot water is a problem.

Turn on hot tap and 80% of time it will fire up.

If i try to adjust the tap, ie use mixer to add cold water, or reduce the pressure the boiler cuts out.

Some times there is a sort of whistling noise coming from the boiler, a bit like a kettle? this can happen with CH, but 99% time happens with water.

Pilot light does not stay on, but lights itself every startup, i found this strange?

The pump is set on II, but there is a plastic box next to it with the same markings (I, II, III), but it is set to I, should these be the same?

When i run a bath there also seems to be some small black specks (1-2mm) appearing, when touched they leave a stain on the bath, it cleans off but i guess they shouldnt be present?

The breather on top of the boiler has alot of build up around it, will this cause problems?

Since i removed the lid & checked tightness of everything i think i may have knocked the "safety releif system", a knob under the pump, it started lowering the pressure, so i tightened it back up, but ever since it has been dropping pressure, oops!

Dont know if these problems all tie in or not, but some answers would be good, i will be getting a pro out, but we got duped last time & would like an idea of what needs fixed before we get someone out.

many thanks!