Passing data to Gravity Forms in marketpress checkout process

I would like to embed a Gravity Form on the /store/shopping-cart/confirmation/ page in Marketpress.

I have managed to do this and it seems to work OK.

Now I would like to pass the following data to the form using a hidden field and populating it dynamically.

The essential data I would like is Order ID, but other fields like name & email address would be useful.

According to Gravity Forms, this data can be passed to the form via query strings, shortcode and/or hooks.

Can anyone give me a clue how to do this?

I should also say that I am using the manual payments gateway, so there is no payment processing in between.

Thanks
Ian