Time machine backup failed: says drive is full but it isn't

My time machine backup is about 1.5 TB on a 2 TB drive and has been happily backing up for the last 8 months

However I suddenly get a message that backup failed, trying to backup 1.5 TB and only .5 is available.

It seems to be wanting to make a whole new backup instead of updating the existing one which it should be able to do.

Also when I go into time machine, I cannot go back into the backups, as if it is not seeing the last 8 months.

Does anyone know how I can fix this?