I know this is PPRuNe, but the level of response has, to say the least, generally been disappointing.
It was hoped that there would be a sensible and balanced discussion about what was a very serious and potentially life-threatening situation (even on this amateur Forum).
This pilot, for whatever reason, not only broke a number of the Rules of the Air but, in my opinion, endangered an aircraft and its occupants.
Talk of precautionary landings and instrument approaches and other such scenarios are out of the question given the position of the aircraft and the atrocious state of the weather at the time.
I was startled by the aircraft due to it's low height above the ground and did not need to 'rubberneck' the registration as it was plain to see in my windscreen by simply moving my head!
My initial thought was for the safety of the occupants and I stopped in the motorway services within a couple of minutes of seeing the aircraft to enquire of Liverpool ATC whether an aircraft was in difficulty, not to report the pilot for low flying.
In view of the comments and criticisms I have received I consider the only way forward is to file Form FCS1520 for an Alleged Breach of Air Navigation Legislation against this pilot.
This is not a laughing matter.
I'll let the CAA be the arbiter in this case.
I agree that much of what has been posted in this thread has not been helpful but have to ask if your decision to file the form is intended to help the pilot or a reaction to criticism of your point of view and thus intended to justify your own actions?
If you are trying to help the pilot avoid a similar situation in the future, and given that you say you know the aircraft and where it came from, surely a quiet word with the pilot or the CFI at the airfield will achieve more than filing the form. After all, if the pilot won't listen, you can always file the form later.
My £.02 worth.