i have seen a few proessional recorder-players playing trills with their left hand - in the music that i am involved with (irish music played on the irish pipes) very few players seem to be capable of this (liam o'flynn and seamus ennis are exceptions) - i have tried it myself but it doesn't come naturally and the fingers on my left hand don't have the same potential for quick trilling movements as those on my right hand - is there a secret to this that i don't know about, or is just brute-force practice?

Unfortunately, the only answer I know is practice-practice-practice. The speed develops over time. What is more critical is playing the trill as evenly as possible. In faster music just do a mordent or inverted mordent.

One hint at the speed of the trill is the overall speed of the music. Trills in slow movements should be more slow that those in fast movements. There is no rule as to how many notes you should squeeze in but focus more on the evenness and clarity between the notes. Always keep the sound elegant.