@RB: I'm not sure why the integral condition didn't appeal to me yesterday... (and sorry if you feel I dismissed your suggestion - this was certainly not my intention). It's certainly grown on me since I had a look at the alternative! Thanks again.

I was rather hoping that I had missed some subtle difference between my question and the one there which meant that mine had an easy answer... I will, however, do as you've suggested. Thanks for your help.

I will certainly investigate Horvath's book. Thanks!

Ah. Thankyou for that! Can you suggest any relevant references?