Looks like something else has gone wrong because I've been charged for the last book I've just bought and yet I should have those £20-odd digi credits left.
I have a feeling it's going to be a nightmare because CS won't have a script for that
Edit: ah no, re-reading the email, it looks like they've credited back as 'normal' promotional credit and not digital credit, so I reckon it should get deducted from the next physical order, rather than ebooks (I think!)
Aaaand yep, if I put something in my basket and then go to checkout, the £21.78 are deducted there.