Jim, Joan Littlewood remembers that story slightly differently in her autobiography:

[1995:] Luckily I was held up listening to Hamish Henderson performing an exegesis on an old Scots song [at the 1949 Edinburgh Festival]. 'She doesn't realise it's another language, with different roots,' he told me, towering above a little lady from California, magnificent in his Highland regalia. '"Johnny gars my tail toddle"- that is to say, "causes my pudendum to rotate",' and as he spoke he illustrated the point by gently undulating his backside, which made his sporran waggle a little. The little lady looked quite pleased, which was reassuring. (Joan Littlewood, Joan's Book 398)