Like everything in this case!
According to JuJ, JoF was in her house with JoJ until after 3.23pm. GD claimed to have left his house around 3.15pm, his mother taking him to Dalkeith for his Jobcentre appointment.
Somewhere in all of this, Jof apparently called GD from his (JoF's) mother's house and was in GD's house before 3pm, in order to be working on the bike. (If he didn't arrive until after 3.15pm, GD couldn't have seen and spoken to him there.)
There is one irrefutable, undeniable fact in all of this - JoF could not have been in two (or maybe three) places at once. So where was he? JuJ's house or GD's house?
If he wasn't in JuJ's house, the reason for the cancellation call to the doctor (that JoF and JoJ were going to be in JoJ's room smoking cannabis) cannot be correct, leaving the question, what was the real reason for the cancellation call?
If he was in JuJ's house, the only way for him to have been the other youth on the moped is for him to have left JuJ's house on receipt of a call from GD from the Jobcentre at 4.20, gone round to GD's house and picked up the bike, then set off for Dalkeith. That would have meant the timing of their sighting at the Tool Hire place ruled out their story of "mucking about in the woods on the bike for a bit" - by the time JoF got to GD's house, got the bike, got to Dalkeith and picked up GD, there would be very little time left for "mucking about" before they emerged in the Tool Hire grounds.
Or, alternatively, he went to GD's house, even though GD was not there, worked on the bike in GD's garden and was coincidentally, right there when GD called at 4.20pm. Nobody ever suggested that was what he did and the only person who would have been able to verify it would have been GD's dad (since his mum was already out and the other friend who was in the house that afternoon left with her and GD).
What I've never been able to get my head around is why the police didn't go back to all of them and say, this can't be right - between 2.30pm and 3pm, JoF was, according to all of your statements, in two places at once. Nope, accepted all of the statements as "fact" even though, as I've just shown, someone had to be mistaken ... or lying.