Hi phs-uk,
To use the send later option tick the box on the invoice when creating. When you wish to send the invoice, you can send this by going to Sales > All Sales > Filter > and change the 'Delivery method' to 'Send later' to see all unsent. You can then batch-send or individually select the invoices you wish to send from here by checking the box and going to Batch actions > Send transactions.
Thanks