It was all working fine for me by the time I got to try again (in Chrome btw), so thanks @Kinetic for sorting it out so quickly.
One minor point: the “cancel subscription” button is very large and evident and I wonder if it could easily be hit by mistake. I hope there is a “do you really mean it”? question built in for such eventualities?
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And a slightly off-the-wall point: I wonder if you could divert the odd £100 of the growth fund into a premium bond. I know the chances of a big prize are vanishingly small, but at least you never lose your money, and there are 12 chances a month year(!). (It’s an investment, not a gamble…). Well, just a thought… 