Andy,
I followed this thread with many interest and have got a big problem ...
Following your informations ( and probably the report of the french mayor ), the ALLEN's P2555 crashed at POIX-du-Nord on 18th may 1940 between 6 h 30 - 7 h 00 AM ( about 12.00 PM for Arnaud ). The date on the tomb is 18th may 1940 dated. The 85th Sqn operated in this zone on 18th may.
I suppose you know this links :
http://ivonov.in/www.dailymail.co.uk...shot-down.html
There is a very interesting telegram sent to ALLEN's family. Have a look at the the little paper upside, dated 17th may 1940 ( "3.17.1940", but you must of course read "5.17.1940", the "3" is a "5" ). Then, this telegram was sent on 17th may 1940 to LEICESTER. Probably on the evening, when the post office of LEICESTER was yet closed because, received at 08.04 AM on 18th may 1940 ( see the two red and black stamps, on the left and right sides ).
Then, how explain the death of ALLEN at POIX-DU-NORD on 18th may 1940 with a telegram sent to his family a day before ? I don't understand any more.
Best regards.
Christophe