“There was a single primary return received (…) 16 seconds before SSR returns were lost. […] No explanation can be offered for its presence.”
Did you know that the RADAR information regarding PA 103 has NEVER been explained?
Here is the key paragraph from the AAIB. ( https://assets.digital.cabinet-office.gov.uk/media/5422f370ed915d137400046f/2-1990_N739PA_Append.pdf )
The following picture shows the last radar returns of PA103. The unexplained return is marked by the rhombus on the right side of the second return before last.
Here are the locations of these radars. ( I am not sure about St Annes. Is it the one next to Blackpool Airport?) The red line is the flight path of PA 103: about 310°.
Also, notice that the debris were spread left AND right – over about a nautical mile – in a few seconds after the last return.