There's a phillips-head/hex screw on the top of the housing facing upwards. Number 23 on this diagram. The drain bolt is 21. https://www.fowlersparts.co.uk/parts...010-a/oil-pump

If you crack that loose half a turn whilst the engine is running oil should start to seep from it after maybe 10-15 seconds. If nothing has come out by a minute then shut the engine off.

Don't take the screw all the way out by the way, you will make a mess!