Just thought I'd update on the original topic here.
There was some suggestion that I was saying RC should support PHP 5.2.17. I was not. The installer was telling me that it was supported with SQLite, and I was trying to make use of that option. I have no problem with RC not supporting < PHP 5.3, although it would be nice if the installer made it clear that it does not (perhaps it does with PostgreSQL which I did not want to use; anyway the installer does not report properly regarding SQLite in the situation I was in). It would also be nice if the INSTALL file could be updated so it is accurate in relation to SQLite.
I upgraded to PHP 5.3 last night, and one of the two sites using older code that I was concerned about is fine. The other remains to be tested by the client who runs it, which I guess will be done later today.
Roundcube is up and running and I am very pleased with it. Thanks for the great software and to Noel and Seb for the advice.
Nigel