How can I stop iTunes uploading my tracks from my iPhone 6s to iCloud?

The answers already posted to questions similar to mine don't seem to apply to my phone. I can keep individual tracks on the phone by pressing the download button, but with nearly 2000 songs on the phone, this seems like a long task, with no guarantee that the song won't simply upload again. Is there a way (as for photos) of preventing it?

Using the Cloud is difficult where 3G/4G reception is poor, underground in London, and it uses up my data allowance very quickly indeed.... However, I do need email and calendar to be up to date, so I don't want to turn off the Cloud entirely.

From my reading of your question, there's actually three separate things you're trying to achieve and/or concerned about.

Email and Calendar

Keeping your email and calendar synced is an iCloud setting. To ensure these are up to date, go to Settings > iCloud and ensure that both Mail and Calendar are toggled on.

Music

If your music is uploading to the cloud then you must have iCloud Music Library enabled, so you would need to disable this. To do this, go to Settings > Music and ensure that iCloud Music Library is off.

NOTE: You should be aware that this will mean you won't be able to access songs, albums and playlists from Apple Music. However, if the only reason you're wanting to stop this is out of concern for your data allowance and patchy 3G/4G reception, then refer to Mobile Data below.

Mobile Data

You can actually leave your iCloud Music Library enabled, but restrict all syncing to WiFi networks instead. You can do this by going to Settings > Music > Mobile Data and selecting what options you want enabled/disabled.