I finally discovered the answer to this problem which crashes the normal installer and requires that the manual install be used.
This problem occurs in some distributions (such as Red Hat (9?)) due to there being an installation of the JRE in /usr/bin/java – whereas my currently active install is actually elsewhere (in my case /usr/java).
The installer seems to be looking in /usr/bin/java first. It probably ought to be doing a ‘which java’ or something instead.
The solution is to delete or rename /usr/bin/java before installing.