"Remember", he said, "All you have to do is blow. And Roll your Arse. Can you do that?"
"I think so. - But I'm not sure why I have to roll my arse?"
"No" he said - "Roll your RRRRs - while you're blowing"
I'd just bought a wooden pipe from a street trader at the Sidmouth Folk Festival. A pipe, or whistle, that imitated a blackbird singing. He'd made it look so easy. It sounded very realistic. A deep breath. A long blow and pull the end of the bird flute in and out. But apparently I had to be sure and roll my RRRRs at the same time. I never did get the hang of it. However much I rolled my arse.