WW2 RAF GREATCOAT REGULATIONS ?

Hello to one and all,

I have been searching the old faithful net for the past few days to see if there was any info to do with the Regulation length on the RAF Officer's Greatcoat, to my surprise I found nothing on this subject. So , is there anyone out there that might know of any Regulations as regards to the official length or was it just a personal preference, eg just below the knee , above the calve or just below the calve, your expert knowledge will be most welcome.

Thank you,

Tony S