That's what they say, rockgran, but I do not think it is true.
If you look at their website, they give a list of the large winners. If you have nothing better to do, you can order the list in area, amount held, amount of the bond, date bought, etc.
It's so obvious that large blocks win most.
Twice we won £1000, but that was when we had the maximum, after we had sold a house or a business and put the money into bonds. One year we got over 7% return, but again that was when we had a large block.
Most of the prizes go to bonds bought since 2000 as well, so if you are cashing any in, cash the older ones first.
My brother-in-law's father was an investment banker for TSB, and he used to get all the spare cash from all family members to buy blocks, any winnings to be shared out at the end of the year depending on the amount of your share.
Another welcoming into Soop's kitchen for those who care to share...