"David (in Normandy)" writes
Since the Lottery started I have had the same 6 numbers. 5 have come up
tonight.

Would you now change numbers or stick with them?

Doesn't make much difference. This is what John Haigh of Sussex
University says about your chances of winning:

"It is hard to appreciate how small this figure is: for example, the
chance of death within one year for a middle-aged healthy man might be
one in 1,000. If so, the chance he dies within the next week is one in
52,000; within the next day about one in 365,000, within the next hour
about one in nine million - still bigger than the chance of a jackpot
share. Indeed, on these figures, he is about as likely to win a jackpot
share with one ticket as he is to die in the next 40 minutes."

For me, what is more telling is that, if I were rich enough to buy one
ticket in every possible combination, and thus guarantee a share in the
jackpot, the maximum I would win would be 25% of what I put in (since
IIRC that is the proportion of the money that goes to prizes).

Statistically it makes not the slightest difference to your chances of
winning if you keep them or change numbers every week - in much the same way
as tossing a (balanced) coin - like getting four heads in a row doesn't
change the likelihood of getting either head or tails the next toss both
exactly 50:50.
The only thing to bear in mind about selecting lottery numbers is to avoid
having sequences that are likely to have also been picked by other people
eg. 5, 10, 15, 20, 25, 30. Such sequences have exactly the same odds of
being the jackpot as a more random sequence such as 6, 17, 19, 34, 42, 49
BUT you would be likely to share the jackpot with more people with the
former sequence. I wonder how many people have silly sequences like
1,2,3,4,5,6?

At one stage about 10,000 chose the first six digits - see
http://plus.maths.org/issue29/features/haigh/index.html.

This article discusses commonly chosen patterns and suggests strategies
for avoiding having to share your win.
--
Kay