also is worth checking that oil pump pick up pipe,

theres a small gasket in between the pipe and strainer, this can sometimes fail and lose oil pressure up to the head, but its more likely to fail all together.

try the cheapest bit first, if its was me i would get a oil pressure gauge and test the pressure, and look at the gauge when the tapping noise kicks in, if the pressure drops at the tapping, its properly the oil not making it to the head

oil pumps arent that hard to change, about 60 quid for a pump, could try this first, or wip the sump off and check out that oil pick up pipe