Hi Andy and Lou,
I've had a similar problem. Went sale agreed on a house 16 weeks ago, paid booking deposit, carried out survey etc but only received contracts from the vendors last week, minus a closing date! In the meantime we've lost the buyers for our apt and have had to put in back on the market. The only reason we haven't withdrawn from the sale yet is that we haven't found another suitable property but we are looking.
The main reason our sellers didn't release contracts was because they hadn't found a new house themselves. It was only after repeated threats that we'd pull out that contracts were issued. Your situation sounds a little sus. Maybe the vendors aren't 100% certain about their own purchase or perhaps the new build is falling behind schedule. You should keep in contact with the estate agent - put the pressure on.