Thanks. I managed to crack it! It appears that I needed my own PHP.ini in the root of my domain. This overrides the root PHP.ini on the webserver which I do not have access to. The settings were:
[mail function]
; For Win32 only.
SMTP = mail.crystaltech.com ; for Win32 only
smtp_port = 25
sendmail_from= admin@primustech.co.uk ; for Win32 only
