1) Login to Webmail: https://mail.zarro.co.uk (select 'Webmail')
2) Select the 'Settings' cog:
3) Click 'Mail', then 'IMAP Accounts' and click the edit/Pencil icon on your account:
4) Next to your email identity, click the little arrow to expand your email profile settings. Within here, you can set a signature.
Click 'OK'. Once complete, ensure that you click the green 'Save' button: