I have never known a witness who 'thinks' they maybe saw [named person] ever making a witness stand- A lawyer just wouldn't allow it. As Martin was 60-80% certain then his testimony would be taken seriously. I am only surmising this is why it was only him, however as you point out, there may have been other requests on paper to invite others in the party, which have disappeared or got lost?
I personally, wholly understand Mr Smiths anxiety, no one wants to get people into serious trouble and he feared his information may do just that-the thought of a missing child must have outweighed that and he let his conscience guide him there fore he came forward.
Sorry, I've only just seen your post.
I have no problem whatsoever that he decided to report his sudden doubt. My problem is the fuss made over it and how, once clarified, some people still have a selective memory syndrome... and that includes a certain someone of the "we policemen, experts..." variety.
At that point, it wasn't a question of taking the stand. A new witness statement would have served the purpose at the time to have something to hand over with a bow to the prosecution.