Think about what *else* you're doing with them, too.
'A UK Software Firm' (*cough*, Data Connection, *cough*), puts candidates through about 5 hours of written exams to fry their brain directly before they get to the programming interview.
Maybe it might be wise (and, indeed, even-handed!) to look at your own interview skills, if you're not getting the results you expect from what appear to be able candidates. I've been interviewed by programmers who - and I can tell by your posts that you're *not* one of these - had all the interviewing skills of a blind molerat. Interviewing really well is so much more difficult than it looks.