laserdisc10, all very interesting points. The funfair story is all a bit of a mystery to me. They got worried about 7pm, and instead of phoning Tia's mum and checking if Tia had called, they instead wasted another hour apparently searching a funfair, before deciding to drive 'literally straight round to Natalie' (I think that's what SH said).
About the funfair, is it possible that SH invented the funfair story so he could explain away one of the hours before Tia was reported missing? Did they actually leave at 7pm to drive directly to Natalie's house, explain what had just happened, and get all their stories correctly matching for the police?
A bit far-fetched, I know, but he did mention exactly what time they left for the fair and how much time they spent searching at the fair, which would be perfectly reasonable, if just one member of staff had remembered seeing him.
So if he didn't go there - what was he doing in that extra hour? The cleaning up was done in the morning if we believe the 'cleaning' part of his interview to be true, so it is possible the pair of them went straight to Nathalie to tell her what had happened. I can't shake the feeling that everyone knew, but were convinced it was a terrible accident and no one could 'tell' because dear old granddad would get into trouble.
It would also help to explain why Tia's mum was so absent throughout. If she knew the truth, her family definitely wouldn't have wanted her in front of a camera for fear of giving something away. And it's not unknown for families to keep dark secrets for generations.