Thanks to Alan for his comments and hints (Netnanny etc.).
We don't have the e-mail problem because only staff have Internet e-mail (on the school ID!); students have to forward curriculum related mail via the relevant tutor and replies come back the same way. They do have internal e-mail from year 12 and up. We may extend this to years 10 and 11 after September but still internal only (using Pegasus).
It is indeed a constant quest to restrict undesirable sites but that is what I (naively) expected of our ISP's filtering. Porno is the one that our students seem to want to try at present, though I have trapped several non-sexual but still inapropriate sites.
What filtering apps. are going to come from the SuSE stable?