Thanks Alec,
I persisted with this and have got marktojunk2 configured and working (well, the button is there and it says it is marking as spam, so all looks good there.)
I am trying to now configure managesieve to add filters. 'Filters' is now available under 'Settings', but I have this error "unable to connect to server" when I click on Settings/Filters from within Roundcube.
At the moment, all I have done is add managesieve to the $config [plugins] as follows :
$config['plugins'] = array('virtuser_file',
'archive',
'zipdownload',
'markasjunk2',
'managesieve',
);
and also changed this line in /managesieve/config.inc.php from
$config['managesieve_port'] = null;
to
$config['managesieve_port'] = 4190;