I bought my first lot of premium bonds in about Nov 06, and my second lot this month.
Comparing the prize band splits, it seems they have changed quite a lot WEF feb.
Lower (£50/£100) has gone from 87% to 70% of the prize fund
Medium (£500/£1000) has gone from 6% to 15%
High (£5000+) has gone from 7% to 15%
Edited to add: IR now 3.8% from 3.4%
