A bit more detail on odds of winning/not winning (if I can remember properly how to do the maths...)
Each bond has a 1 in 24000 chance of winning according to NS&I
A single bond therefore has a 23999 in 24000 chance of NOT winning, a probability of 0.999958333
If you hold 30000 bonds, the probability of NONE of them winning is 30000 of these 0.999958333s multiplied together, which comes to 0.286497336 or 29%
Therefore, in any month, the probability of this NOT being true (i.e. of winning at least one prize) comes out at around 71%.. I think