Its roundabouts and swings, if you get 30000 on block then you only have one prefix number (I.E. ABC12345678) and if that prefix doesn't come out you have no chance of winning that month, however if it does come out then you have 30000 chances, on the other hand if you get 300 blocks of 100 (over a period of time) you will will have lots of different prefixes and a better chance of picking one up in the draw, but with only 100 numbers to that prefix much less chance of a winner. as I said roundabouts and swings. Also I should point out that if you get �30000 worth of bonds in one go they will come in �5000 bonds but the numbers will run straight through.