Hey Adele,
You might try using the "checkboxes" field type instead of the "checkbox" field type on your form. The checkboxes field type allows you to enter 2 bits of information:
1) Display Value -- this is the text to the right of the input
2) Stored Value -- this is the value that gets stored on the record
Once you setup a new checkboxes field, you can select the "Values" link in the right column of the editor to adjust both what the user sees and what gets sent back to the server. This sounds like you a situation where you'd want to add:
1) Please tick the box if you don't want to receive news, updates or product information from MHR International to the Display Value
2) No to the Stored Value
I just posted a walk-thru for changing a "checkbox" field to a "checkboxes" field on another post here: https://nation.marketo.com/t5/product-discussions/make-label-wrap-with-two-column-checkboxes/td-p/315230 and there's some screenshots there of where/how to update the Display Value and the Stored Value for a "checkboxes" field.
Let me know if you've got any questions on how this gets setup, I'd be happy to help -- or if you were able to get this working for anyone else who might run into a similar issue in the future.
Thanks,
Dave