Before you reserve
Confirm the developer owns the land outright and that the plot carries no mortgage. Ask for the planning permit and building permit by number. Both are public records.
At contract
Deposit your contract of sale with the Land Registry. This protects your interest in the property from the day it is lodged, well before title issues. Engage an independent lawyer, not one recommended by the seller.
At delivery
Snag the property properly and hold retention until the list is closed. A developer confident in their build will not object.