Google Checkout as checkout option, not on shop pages

I have finally managed to get GC to show up on my shop. However, I want it as one of the options on the checkout page, rather than as a purchase option on the various shop pages.
I did previously have this showing as an option on the checkout page, but it was only a listed item, not a button: I have followed the threads for the buttons, and changed all the settings to get a real button. Now I would like to have the button shown only on the checkout page, along with PayPal & the other options.
I apologise if this is somewhere in the great long thread that has been going for 2 years...

I am using ZC 1.3.8a, with PHP4.4.8 and have GC 1.4.7.
I know I will encounter a problem moving from Sandbox to Real, as I don't currently have SSL, but that's another days' problem!
Tia for any help.

Mark.