Catch Up is only available for 30 days since broadcast date. Episodes 1-5 are, therefore, correctly not available on Catch Up. The first episode, currently available, is episode 6 but it, too, will soon become unavailable.
Re: Missing episode: The Truth About the Harry Quebert Affair
Catch Up is Catch Up for currently broadcasting series/programs.
On Demand is for material not currently being broadcast and availability is down to agreements between Virgin Media and its suppliers. There is no guaranteee of when or if this material becomes available.