I LOVE London Broil. Grew up on it. After ruining way too many of them, I started using a meat thermometer. I've got a great digital one you can get for less than $20. I don't know how much the regular ones are, but most supermarkets sell them. Make sure to get one because I could tell you to cook it for about 20 minutes at 350 because that's what mine usually go for, but For your cut it could be totally wrong. how thick is your cut? 1 inch, 2 inches? less? more? Makes a huge difference, so get the thermometer. I cook to 120 degrees because we like it bloody. YUM! Sliced thin with an electric knife, cooked like this, it's always tender. Go to about 130 for a nice pink.
Yes, marinade is important. I go very simple. Prick the thing with a fork and shake on Worcester sauce on. Let sit half an hour, flip repeat. This also gives it a chance to get to room temp so you get more even cooking. Also, if you have a rack cook this on a rack. When you do this, you don't have to flip it during cooking. If you don't have a rack flip at about 100 degrees. I actually use the rack that came with my toaster oven.