Sorry, I've no idea who the parents are for the 1818 birth/baptism. I don't have a FindMyPast sub. The image may be just the GRO index list with ref numbers, should you want to order the record from the GRO. Seeing the actual GRO record is the only way to be sure if it's your Bell family or not.
Perhaps someone here with a Find My Past sub can look at the image for you, and confirm if its an index page. Ordering a copy from the GRO is just slightly different than ordering the usual BMD certs, but it wouldn't be in PDF format. You'd get it via regular post.
Wasn't your chap discharged in 1818? Depends how long he was in SA for, before he went back to England. The 1818 birth/baptism in question could be from before he was discharged or just after. That's if the birth/baptism is anything to do with your chap, obviously.
