2005 Citroen C4 low oil pressure

The car comes up with Low Oil Pressure and a stop on the dash ( when oil is warm). The sensor, oil filter and oil has been changed.

Noticed that the car had a bit of shaking although misfiring, so replaced coil pack and the spark plugs.

It also has had the rocker cover gaskets replace due to leaking.

It has a tappet sound when first started up, then it is ok after a while, but when I go to stop then the Low Oil Pressure comes up.

I know that the Cam belt kit is due to be replaced, and is booked in, and I am planning on getting the water pump done at the same time.

The engine does whir when acceleration is applied, sounds like it maybe water pump or the oil pump.

Does this sound like it is the oil pump that needs replacement?
I usually order the parts in, as I can get them a lot cheaper from abroad, and are genuine Citroen parts.

It is to give me a clue to how costly it is to do, I am not planning on doing it myself, just get an idea of what is involved!

Thank you Nick