Repeated with necessary corrections . . .
SUCCESS at last ! I confirm that the LG BP40NS20 6x burner is ocmpaible with the 27" i7 iMac with OS Lion, at present being sold by WAE on line at the cheapest price of £73.18.
Several BD discs successfully burnt so at last I'm happy with my system. Hitherto I've been editing in FCP X then transferring the finished film to my 32 bit 2006 MacPro 1.1 for burning on its internal BD burner.
Durng my period of searching I many times came across reviews that confidently praised a burner but the reviewers failed to fully descibe their systems . . . Those who do reviews of any sort, please take note - it's annoyingly frustrating !