Hi, I want to go to london next month, but I was refused entry 4 years ago but I was under age (was 15), do I need a visa to go now, or can I just go without being worried? Or if I have an invitation letter do I get in no problem?

What is the purpose of going to London? Are you going on vacation, or going to work? What do you mean by an "invitation letter"? How long are you planning on staying in London?