Melatonin

Hello
I'm new to this and think I have DSPS. My natural cycle is to sleep at around 3-4am and wake at around 1pm.
I have heard of a drug Melatonin. I am in the UK. Can you buy it? or must it be prescribed?
I am afraid to just ask my Doctor in case he says that I can't use it without tests because I have been in and out of Hospitals and testing centres for years and they see nothing. People just tell me that it isn't a big deal and to just get up earlier.