First class upgrade from 2nd if free shipping?

Hi,

I need our zen-cart shipping to let the user select First class post when they have an order that is free of shipping.

Since royal mail changed the way they price sizes of packages we need to be able to send by 2nd class as the default as even our cheapish products can be quite large! The users need to be able to select a first class tick box or similar.

I'm a competent php programmer but obviously dont want to change the code unless there is a way to do it with the current modules.

If i need to code it can someone please talk me though what files do what? :bigups:

Any ideas chaps and lasses?!