One interpretation of the rules is that you would have to wait until 0930 for the delayed 0900, rather than being allowed onto the 0915. This interpretation is where the 60 minute rule comes in to play - if the 0900 wasn't expected to leave until 1005 then you would be allowed on the 0915.
I don't see the logic behind this interpretation. The next available train following the delay is the 0915. Nor indeed am I any wiser as to where this 60 minute rule has come from. There is nothing I can see in the ticket's terms and conditions which specifies I must wait for at least 60 minutes before catching the next available train. They just say "the next available train(s)".
I do not know if this rule is actually written down anywhere.
You won't find it written, it's a custom and practice thing, or internal Control guideline.
Just as well I can rely on the written rules in this scenario then, which quite clearly permit me to take the next available train.