Hello again!
I've managed to get scans of the Movement Record Cards for PL896 and PA893 from the RAF Museum London…
http://www.rafmuseum.org.uk/london/
They're on my blog…
http://hewasahero.wordpress.com/
…and here we see the dilemma crystallised. Planes with both these serial numbers didn't crash on 20 September 1944, so there's an error in one or other set of record copies (and almost certainly in the original records). At first glance, the number of entries on the Movement Cards gives weight to their being the more trustworthy source, but if they are correct, then where did 'PL896' come from in the Operations Record Books? It doesn't look like a mis-spelling of 'PA893', and just happens to be the serial number not just of a Spitfire, but of a Spitfire of the same model!
Who can unpick this Gordian knot?