I'm confident from what I read last month that the pic changing doesn't mean you've won this month (think it only changes if you had a win last month).
No error message for me on the blue app on transations @OneFrenchEgg Also - probability means there will be less low value blocks winning a prize. If you have £5,000 worth of bonds, made up of 200 x £25 blocks, you will have the same chances of winning as 1 x £5,000 block. If you withdraw and buy again, you just miss out on a draw. I'm saying that as someone with only a low value of bonds. Interestingly the one I won on last month was only a £35 purchase.