Limit to "Exclude programmes" list length in Web interface

[BigG]

My current "Exclude programmes" list length is 648 characters. If I add, say, "Perfection" to the list, the search returns no programmes. If I remove "Perfection", search works fine. I'm obviously hitting a limit (650 maybe, but who cares?) which I would like to remove. Does anybody happen to know where this limit might be. I'm guessing that the HTTP request gets through intact (I'll sniff with wireshark a bit later to confirm this), and that either the bit of Perl which parses the request, or the bit of Perl which does the DB filtering is the problem. If anyone happens to know, it'll save me hours of debugging......

BillG