Help L series slow oil pressure

I am an experienced mechanic, but hoping that someone has come across this problem before and can save me some time. If I start the engine ..say the day after a short run (ie engine cool), the oil light goes out straight away. If I start the engine the day after a long run (ie engine hot), it can take over 30 seconds for oil light to go out. Oil pressure appears normal when hot and no heavy knocks. It appears that there is an air leak on the suction side of the oil pump allowing the oil to drain from the pump when hot. As the pump is on the crank, could it be the crank seal but no obvious external leaks, or is there a seal between the sump pick up pipe an the pump that can give trouble.

Hi I have the exact same problem. What was the outcome?

I took the oil pump off and the housing was cracked. A new oil pump and gasket was £200, cheapest place I could find was Famous Four.