Just got this offer via e-mail.
Available on c2c service.
To avail this offer you will have to fill out the form online [link given] following which you will be sent the voucher to your e-mail.
You need to purchase one off peak return adult ticket to your destination [on a c2c service] and your companion travels free. If you are a season ticket holder, you only need to produce this to allow your companion to travel free.
Great for visits to London.