Is it fenced in and has it been for twenty years? Then you can apply to the Land Registry to register a claim to it, but I'd be strogly inclined to leave things as they are because, once you register the claim, Persimmon will be on notice and may do all in their power to resist it. The longer things stay as they are, the better your claim, or that of any subsequent buyer. And don't write to Persimmon, or they'll definitely start arguing and start claiming a price to convey it.
This happens surprisingly often. I developed a site of 40 houses. Many years on, I discovered that the solicitors hadn't conveyed a fairly substantial piece outside one plot, by the roadside, to anyone, and I was still the owner. But nobody had fenced it off. I assume that it was meant to go the plot owner. Anyway, the current owners of the plot have now offered to buy it. And my current solicitors are not in the least surprised by the original mistake; they've had it before.