Henk;
The 6 Group records at
http://www.6grouprcaf.com/ confirm that Williamson was the pilot on the flight where Bertrand was lost, but doesn't give a serial. From memory, it was a post at RAF Commands Forum that linked Williamson to HE239, but I can't locate it now. I have also seen HE158 reported with the rear turret blown off, but without a date and without any casualties listed.
Richard;
Can I ask your source linking HE239 to Williamson and Bertrand?