customise message after 'payment by cheque' selected

[LouPhi]

My customer wants to add a message to the redirect page when a customer opts to pay by cheque, but I cannot see what I need to edit in order to enter his preferred text.

I know in 'design | text' I can edit existing text messages, but what my customer wants to add is alot more than a couple of words.

Does anyone know how/where I do this??

Thanks