Redirect Offline payment to custom page

Hi there

We use the offline payment method with one click checkout. The current process is

1) User choses a plan
2) User registers and selects 'Offline Payment'
3) User then gets the ofline payment form i.e

(root)/plugins/payplans/offlinepay/app/offlinepay/tmpl/form.php

4) They choose cash / cheuqe etc
5) They choose and then they are taken to the thanks screen that has a copy of their invoice

We need step removing, so the offlinepay plugin need to redirect to a custom location of our choice (an instruction page)

Can you advise where this redirection exists in the code so I can intercept it and our own redirect (JUST for offline payment)

Thanks