I think I have worked out what happened. There was one original farm with a farm house and lots of surrounding land. The older couple buy the farm house, and they would also like to buy some of the additional land but not all of it, but they don't have enough money yet. In particular they want some paddocks. The original owner Bryn initially says he will wait to sell the land so that the couple can buy the paddocks they want, but then he changes his mind because it's too much paperwork. The crazy Collins offer to buy ALL of the land, including the paddocks, and Bryn agrees. They say that once they've bought the whole lot, they will sell the older couple the paddocks, because they are seemingly willing to partition the land up into smaller lots whereas Bryn isn't.
When they're about to purchase the land Collins says to the older couple I'm 10k short, can you lend it to me and it will be a deposit on the paddocks. That is why they agree - they're not just lending 10k out of the goodness of their hearts. It was because they thought it got them one step closer to owning the paddocks they wanted. The land sale goes through so Collins now owns ALL the land around the house. The agreed cost of the paddocks was 25k in total, so they then send the additional 15k and wait for Collins to sort out the paperwork. He then says he doesn't want to sell it to them after all and it seems like at this point it turns sour but it's not entirely clear why. Perhaps the older couple felt deceived and like it was a ploy all along. I'm not sure what Collins actually wanted after this - maybe he didn't want to pay the 25k back and claimed it was a gift or something. At first I wondered whether what he actually wanted was to buy the farmhouse so they could live the life that the older couple were living, which would make the initial fawning etc make more sense. But it doesn't sound like they ever had any intention of selling their house 4 miles away so not sure what the end game was, or if there actually was one or he was just a total nutter with no plan.
Obviously the main error the older couple made was not using a solicitor to purchase the paddock, but presumably they were trying to avoid fees and save everyone money. But they weren't just lending them money - they were trying to buy the paddocks.