You can buy a set of splined keys for around 15 quid.
You ideally need a couple of long socket extension bars too as they are a pig to get to from underneath the car as the driveshaft gets in the way, so it's easier to do them from the side of the car and turn the driveshaft so the bolt you're undoing is at the top.
Make sure you tap the splined socket right into the bolt tight as they round off VERY easilly. May need someone to put their foot on the brake while you undo them as they're usually pretty tight! Make sure you put a little thread locking compound (locktite) on the bolts when you put them back in...... you dont want them coming loose!