You can find your order history by signing in to your online account and going to 'My Orders', provided all orders in question were placed while logged in.
You can also download and print copies of your invoices from your order history.
After you sign in to your account, click through to 'My Account'.
Choose 'My Orders' from the menu; this will display your order history and invoices.
Just click the 'View Order' link next to the order you would like to view.
If you click on the 'print' button, you can also download a copy of your invoice, which includes a VAT total for the order if applicable.