I have been advised by Support that it is not possible to edit the layout of your default Statement template, or to create a new statement 'Theme' as one can do with invoices and quotes.

The standard template includes a field for 'Purchase Order', which is not really relevant for us and is always going to be blank. We would much prefer to replace this with something like the invoice 'Description' field. That would be more normal for the itemisation of an account statement.