Had the "STOP OIL PRESSURE". & "OIL LOW"

light on today whilst driving, obliviously panic stations

Stop car fortunately I had oil in boot topped up but when I open the oil cap I could hear the engine tapping

Even more stress

Straight to garage and they identified i had put too much oil, in reference to tapping luckily they had another x5 we opened cap and sounded same is this normal? Guy said its due to chain cam belt

Anyway they did a full oil change on it using ll04 spec oil

Got home scanned car and have a oil pressure sensor error cleared it drove around and scanned again same error has come up

Question is

Sound car told like that when oil cap removed?
How much roughly is oil pressure sensor and how easy is it to replace?

It's a 3.0 diesel 2004