Okay, I've spoken with DrByte regarding the issues with the Realex mod and it looks like a fix might be to tell Realex to use the following as a callback URL (instead of before_process.php)

YOUR_WEBSITE_ADDRESS/index.php?main_page=checkout_process

I haven't had a chance to try this yet- I'm away from the server that hosts my sites that use Realex. But I'll let you know how I get on when I do.

In the meantime, I'm updating the mod to remove all the legacy code. I will have an update shortly.