This is how I do it every time:
Pour about a half cup of olive oil in a gallon ziploc bag.
Add a heaping spoonful of minced/crushed garlic
2 tablespoons of ground black pepper
1 tablespoon of salt
2 tablespoons of worcestershire sauce
Drop the london broil in the bag, squeeze most of the air out of the bag and zip it closed.
Work the meat around in the bag for a minute or so then just let it sit for a couple of hours at least. I'll usually do this in the morning if I am going to grill for dinner. Make sure to take it out of the refrigerator an hour or two before you plan to cook so that it can get up to room temp.
I usually have my grill set up to 400 degrees for this. 4-5 minutes on each side, then let rest for about 5 minutes (that will give you the med-rare to med. that you see in the picture). You can cook longer if you want it more done, or go higher temp and shorter cook if you like a better char on the outside. You may have to adjust for a larger or smaller london broil. This one was 1.64 lbs.
Slice very thin across the grain. I typically serve with horseradish or A-1.