Step 1:
Empty your cupholders and lift up your center armrest like shown.

Step 2:
Place your fingers in the location shown and pull up, you will feel the panel come up as you pull.

Step 3:
Lift up and remove the cupholders over the handbrake and place to one side.

Step 4: You should now see your handbrake assembly as shown here

What you are looking for is this 10mm bolt.
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Step 5:
I found the best tool to adjust the handbrake was a 10mm ratchet spanner but if you dont have a ratchet one then a normal one will do just fine!
The way to adjust it is to have your foot on the footbrake and release the handbrake, this will allow you to place your tool of choice onto the nut and do it up, keep checking how far the handbrake can be pulled until you are happy!
Step 6:
Once you are happy with how your handbrake sits when engaged slide the cup holders back over the handbrake making sure to place the front of the cupholders in first, like shown and push down.

Step 7:
Once you have pushed the cup holders down and into place it should look like this!

Well done! You have successfully adjusted your handbrake and you only needed one tool!
I hope this has helped, let me know if you have any questions!