While installing the above I found the following obscure error:
The environment under which I run Mantis is a little broken I think and so svn can only work if called with –config-dir /home/svnuser/.subversion otherwise it tries to read /root/.subversion which of course fails
Even if the above parameter is set in configuration the svn_check function appears to ignore it and calls svn help without it. Unintuitively svn help fails if it cannot read the configuration folder so the –config-dir must be applied here too.
I found this by replacing the svn binary with a wrapper that recorded the parameters it’s called with. Adding the necesarry code to svn_check to use config_dir fixed the issue.