[gottago]

Surely costs more to keep a set in storage for 5 years than to just start from scratch too? Considering another change in production company and a change in format this time I'd be surprised if it was the same set.

Hmmm... not necessarily. Keeping a flat pack set in cheap storage for an event that definitely will happen at some point over the next five years - potentially much sooner (2019), and called at short notice (6 week warning this time), probably is significantly cheaper than commissioning a set designer to work alongside lighting designers (and probably many more people) to design, build and test a set with a short turnaround. You're also risking availability of these people doing right at the last minute as well.

The production company changed from 2017 to 2019 but used the same set. It's possible the set is owned by C4 itself for this reason.

It's normal for a production company to pay to keep sets in storage until a show is officially axed and there's absolutely no chance of it coming back elsewhere as a set design and build is a bigger cost than what they'd lose due to storing a set that instead needs to be scrapped.