I cook my sirloin very simply. I buy from a very good butcher and make sure it is thick - 3/4" minimum, roughly weighing 12oz.
I take it out of the fridge at least two hours before cooking.
Melt butter and a little olive oil in a frying pan and when it is hot season steak with salt and pepper. Fry for 3 minutes one side, turn over, fry for two minutes on the other. It should be nicely browned on both sides now.
Put on hot plate and leave for at least two minutes.
If you prefer your steak more cooked through, turn the heat down and cook for a further minute or two each side before taking out to rest.