I just realised
That I failed to take into account the possibility that there *are* no other claimants to the jackpot. In that case, you win the same amount as with one ticket (less the extra quid), whereas buying two different tickets gives you two chances to win the Jackpot, so that's a significantly better (while still very small!) expected return.
( flatfrog Staring vacantly into space, Tue 12 May 2009, 13:00, Reply)