To book it in you need to either do it online. Click herer for the website
What you do is enter the model and serial number of your appliance and then fill out the form and they will contact you to call out and fix it for you.
Or you can ring them up on 08448 224 224 and give them the model and serial number of the sticker on the inside of the door and they will book it in for you from there.
If you get stuck then just get back to me.