Not quite
The 50p goes to the prize fund - a large chunk of this is payable to the jackpot winner, but most of it goes to other prize winners.

If nobody wins the jackpot, only the jackpot portion (32%?) rolls over. So, assuming exactly the same number of people play the next week, the expected return is something like 66 pence in the pound.
(Vice Admiral Sir Charles Cockbrush KCB MBE DSO, who is finding it increasingly difficult to log into b3ta.com wants to duck your aunt and kick your puppy, Mon 11 May 2009, 10:49, Reply)