So I have just found out that the 25% bonus from a Help to Buy ISA does not actually contribute towards the home exchange deposit, it only contributes to the mortgage deposit at completion.
We had been counting on the fact that the bonus contributed to the 10% (or 20% as it is these days!) deposit. But it doesn't. Which is so stupid because it is literally called a Help To Buy ISA. When in fact it doesn't help you to buy at all.
Instead we should have opened a LISA but we can't now, as we're too old!