When I end a note on recorder the pitch falls. This is great for certain styles of music, but on others it just doesn't get it. I'm looking for a nice clean release. I've tried a sharp blast of air which doesn't give the sound I want, and I tried cutting off the sound with my tongue on the roof of my mouth - this gives the sound an unmusical thwack! From the research I've all ready done it sounds like the last method ought to work, but I can't seem to get it. How can I do this?
Tonguing at the end of the note will help you to stop the flow of air, and hence the sound, quickly and cleanly. A lot of recorder methods/teachers recommend tonguing at the beginning and end of every note (unless you're slurring of course) so that you're almost saying "dood" every time. Personally I don't think that's always desirable, and can lead to a bit of a staccato sound, but it's not necessarily a bad habit to get into to begin with.
Don't use the whole roof of your mouth, just press the tip of your tongue against the alveolar ridge (hard lump behind your teeth) rather than the hard palate, and never tongue the mouthpiece. Any "unmusical thwack" sound you hear is probably only audible in your head: bone conduction to your ear makes the sounds in your mouth seem very much louder to you than anyone else. Try recording yourself playing and see if you can still hear the thwack on playback.
However, if you're keeping consistent breath pressure the pitch shouldn't really be falling at the end of a note anyway. You should be able to end a note cleanly using diaphragm control alone if you want to, as you would with a brass instrument. Perhaps you need to work on your breath control more generally too?