Script to add £20 per week to a field

Hi, I am writing an automatum that will add £20 to all records in a certain field every week. I need to (or I think I need to!) write a simple script that will add that £20 to the certain field, just wondered if anyone could tell me how to do this please.

You could have a formula field that adds £20 to the field paired with a weekly automaton that copies the value from the formula field to the editable field.