Thanks for the info A Andy but unfortunately the CAA safety sense leaflets aren't an official definition of anything. They are certainly useful publications, and well worth reading, but it is important to remember that the author occassionally stated opinion as if it were fact.
I regret, that I cannot find a definition of a "Standard Overhead Join" in the Air Navigation Order. I'm not saying that there isn't one, just that I haven't found it.
I congratulate the originators of this thread for asking the question. I hope someone can find an official definition, then at least we will all be trying to do the same thing.
My personal opinion is that overhead joins often make good sense at strips with no radio or for non-radio aircraft but at airfields with radio I don't think they enhance safety at all. Just my opinion.