The dealers will tell you the mioney will run out in October, for obvious reasons. The SMMT figures suggst 118,000 cars REGISTERED under the scheme by end August. Of course this doesn't include cars on order/waiting to be registered. Add on another 60k cars, and you'll find the scheme has plenty of life left yet.
You should be OK for December 15th order date, but bear in mind the further 3 months waiting for delivery - thats another 6 months in your current 'clunker'