Once your public SSH key is uploaded, you can grab FileZilla, an OpenSource file transfer client from here: https://filezilla-project.org/download.php?type=client
Once you've opened FileZilla, click on the little icon in the top left hand corner. This opens a 'site manager'. Click 'New Site', and fill out a descriptive name for it.
On the left hand site, fill out the following details:
Protocol: SFTP - SSH File Transfer Protocol
Logon Type: Interactive
User: Instance name (bit before mytimpani.co.uk domain)
Password: <leave blank>
Once those are entered, click 'OK' to add the details. Next click on 'Edit' -> 'Settings'. Under 'Connection' -> 'FTP', click on 'SFTP'. Then click 'Add keyfile...' and browse to your private SSH key. Click 'Open' to add it. If asked to convert to FileZilla supported format, click 'Yes', and select a location in which to save the new key. Note: this will remove any SSH passphrase.
Then click OK.
Clicking the little dropdown arrow next to the Site Manager, you should find a list of sites. You can now click on the descriptive name, for the site you wish to connect, and you should be automatically dropped into the relevant wp-content folder.