Thanks. I took your advice and tried logging in directly, I get "Connection to the storage server failed"
Checked the log and it says:
Could not connect to mail.server.co.uk:143: php_network_getaddresses: getaddrinfo failed: nodename nor servname provided, or not known in /Applications/MAMP/htdocs/webmail/program/include/rcube_imap.php on line 191 (POST /webmail/?_task=login&_action=login)
I gather that I have not entered the server name in the correct place. I tried looking at line 191in the rcube_imap.php file but I was unsure what to edit.