And I'm assuming you added an extra zero on the second dose.....or did you take a different pain killer?
So I am really worried;
I took a **50 **mg pain killer at around 4 pm, I realised it was not making my head feel any better so I took a **500 **mg pain killer 30 mins later.
I don't know how it works in the UK, but usually you can call a doctor/nurse help line and they can guide you. If you are really worried, call poison control.