Just fixed my loose front arm rest. To do this you need two thin hooks, I made these out of cat hangars and a pair of pliers.

The arm rest is held in place by a bolt which is accessed through the rear air vents. You may need some gloves to prevent hurting your fingers. With a torch you can see in the air vent, either side is a small hole near the bottom. Slide, in turn, a hook and push into the hole. Then you need to pull both, wiggling to remove the vent. It is held in place with metal clips which are very strong! Unclip the connector and remove the vent.

Next, push out the bin underneath which will give you more space.

Unclip the vent hose and you should see the bolt at the rear if you remove the vent. Tighten the bolt (think it's 13mm), put everything back together, and done.