Final checkout steps (orders)

[Dramatic]

Hopefully this is my last question on the orders module!

At present, after entering billing and shipping addresses I'm landing on a page which only says "A required parameter was missing" I can't find that text string anywhere in the commerce modules.
I have reset all prototype templates to factory default after module upgrade (and generated new default templates from them)
Despite this error, orders are appearing in admin, but no emails are being sent (and the cart isn't being cleared).

This shop has
- no taxes (small business below sales tax threshhold)
- no shipping (None of the shipping modules worked appropriately so I created shipping products and the logic to offer them on the view cart page)
- no online payment gateway - payments accepted are cheque or Internet banking (or cash if they are collecting the goods) - The invoice should be presented and emailed to the customer automatically at the confirmation stage.

It would be good if the customer could indicate which offline payment method they will use, so the vendor knows whether to watch the mailbox or the bank account.

[Dramatic]

Update: After kicking Editplus so that "Find in Files" does not hide the results, I've traced the missing parameter to be $order_id

Now to figure out whether it isn't being generated, or isn't being passed through the address form.