How do I add my block of html to the order confirmation page?

[Gary]

I am using the built in Paypal payment plugina nd a Worldpay plugin.

The order confirmation / thank you page after payment is terrible.

I have a block of html within a thank you message in that I need on this page.

I am aware that I need to put this in the plugins/payment/paypal/paypal.php  and the worldpay equivalent, but where?

Everywhere I put it brings the whole site down with an error.

Any advice appreciated, after 4 hours, I still dont have the answer.

Thanks

Gary