I wasn't sure if I needed the brackets or not so I had tried both with and without the brackets and I still get an error message. I just tried again using
S3 end-point: sage-garden.storage.googleapis.com
S3 access point: XXXXXXXX
S3 secret Ket: XXXXXXXXXXXXX
S3 location: sage-garden
and got this error message...
S3 settings test result: Failure: We could not successfully access or create such a bucket. Please check your access credentials, and if those are correct then try another bucket name (as another S3 user may already have taken your name).
The error reported by s3generic was: UpdraftPlus_S3::putBucket(sage-garden, private, ): [411] Unexpected HTTP status