Except....
If I recall correctly, 50p of the pound goes to the jackpot, 50p to the lottery fund/taxes/comission, so the expected winnings is always 50p (given people playing each week).
The expected winnings on a single rollover is around 75p, since the rolled over jackpot adds to the prize fund (it ends up being somewhat less due to the extra interest).
( Tedium Alduce me to interlow myself., Sat 9 May 2009, 16:27, 2 replies)

Even if the maths isn't right,
the sentiment is.

*click*
(Ellinikos contains a source of phenylalanine, Sat 9 May 2009, 16:45, closed)

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, closed)