Although photos from the 2011 sale show a perfectly good battery carrier installed, at some point since then the battery has been replaced and the battery carrier has gone missing.
BMW Classic do not list a lot of electrical parts for this car but they do list all the parts for the battery carrier. However, they are expensive and only fit a battery size which appears to no longer be available.
Fortunately the off the shelf universal battery straps fit. They are not a perfect fit but do the job properly.