Tern Link B7 folding bicycle; unsure how much work required but definitely some

Please note I am not anything close to an expert on bikes so this may be in much worse condition than my description suggests.

Got it off ebay a few years ago and the seller sent it in crazily bad packaging (basically one layer of bubble wrap and some paper). Tried to repair it myself but never got the gears fully sync'd correctly, not sure if that's a limitation of mine or something that got damaged before I received it; think it's possible I left two many links in the chain too. It's generally fine but the chain has fallen off enough times and I done clumsy enough of a job with the wire adding the new derailleur (upgraded it from a Tourney to an Atlus) that I think it needs a full service. As far as I can see the new derailleur has no issues itself.

Think it's possible this is a very easy fix but also think it's possible there's something fundamentally wrong with the frame itself.

We moved to a location with a lot more space very soon after so have no real need for a folding bike.

Some rusty points on bike, none of which have gotten worse over the time we've had the bike.

Collection only but located pretty central (near Old Kent Road). Very open to offers from established buyers. Available most evenings for collection but will need to agree on a time prior to sale

Listing today on October 11th, there is an auto reject in place, I will gradually drop it and the price over time if not sold.