For Aitch..
Well the vid thing was a cctv film.
David is saying he saw everything and is going to pose as Tracey's star witness in her defence.
It's a lie and he's blackmailing her but in return he's hoping she'll give him his first teenage kicks, so she's being all lovey dovey with him on the qt to keep him sweet and he thinks he's on a promise.
They met at Charlies flat (nothing happened) attached to the builders yard and when they left they were looking rather intimate and Jason saw it on the cctv. He was suspicious and showed Deirdre.
She thought it was v v odd too but didn't want to think the worst, and for a while bought Tracey's story that she was just comforting David cos he's scared of the trial and what he supposedly 'saw'.
Of course since Tracey confessed all the other night, she now knows differently, and it's another example of how devious she is.