Gear slipping after sprocket and chain replacement

Hi all,
A few weeks ago I replaced the chain and sprockets on my 2020 M6L Brompton as I felt it wasn’t changing gears as quickly as it used to.

Since then, the lower (-) gear has started to slip when pedalling. I’ve removed the sprockets and refitted them and adjusted the limit screws on the derailleur without any difference.

I’ve been hoping it would improve with time but I’ve cycled about 100 miles in that time without change.

Does anyone have any advice before I try to buy an entirely new derailleur or sprocket set? I imagine the chain isn’t at fault as it only happens on the one gear.

Video of the chain hopping: https://youtu.be/MosW5XLDvW4?si=zki7pgtZeaNtCULu