So I'd replaced Google Photos Backup with Google Backup and Sync but then, even though I was using "High Quality", my photos used up all my free 15GB allocation.
After much reading of Google support articles I finally worked out that I'd somehow put the photos inside the Google Drive folder on my laptop.
The trick is to use Google Backup and Sync to back up your photos but keep them out of your Google Drive folder. Right-click the Backup and Sync icon in the Notification Area and select Settings > Preferences. Make sure, under your Computer, you select your "Pictures" folder (or choose an appropriate folder) and select "High Quality" if you want to use the free storage. And I'd recommend selecting "Upload newly added photos and videos to Google photos".
Then go to "Google Drive", select "Sync My Drive to this computer" and check the folder location is separate from where your photos are stored and you can use up to 15GB of free storage.
I found some of my photos couldn't be converted to "High Quality" and did use some of my 15GB quota, but it was only a few MB.