I have a complex problem. The hello clean checkout does not work with the sage pay interface. The sage pay interface works with the default theme, and the modern theme and the blank theme but not hello clean.
With the hello clean theme the Block HTML cache must be switched off for the interface to work at all, and the interface intermittently gets the wrong order amount (i.e. it does not always include the shipping amount.). The failure ONLY occurs if it is a returning registered customer; if the block html cache is enabled AND the helloclean theme is used.
The sage pay interface people say that this must be a hello clean issue as it works properly elsewhere. This theme is useless to me if I cannot reliably accept payment.
I have finally narrowed down the problem to the checkout.xml. If I delete this file the checkout works and the interface works properly for all customers and with all cache enabled. i do not know enough about coding to look into the xml to see where it goes wrong. So currently my site has a rather sad looking checkout, but AT LEAST I can now take credit cards!!!