iPhone 5 charger just set on fire at desk...

Contact Apple using www.apple.com/support/contact if you are in a country that supports it or call the appropriate phone number on this page. Alternatively carry the charger in to an Apple retail store or service provider.

I can pretty much guarantee they will want to hear about this, because it is at least potentially a safety issue. Whether they will replace it or not probably depends on the warranty status.

I probably don't need to say it, but DO NOT use this charger again.

As you say you are in the UK take it as provided to an Apple Retail store genius bar .

The first iP5 shipments happened just over a year ago so strong chance you have cover (12months warranty)

depending how long you have had the iphone 5

just had my charger spark up on my kid, apple store replaced it, I wish they could put the skin back on my kids finger too

You need to get an electrician to fix the clearly dangerous mains outlet the USB adapter was plugged in to. Turn it off and don't use it until an electrician fixes it. Either that, or it isn't an Apple adapter, which completely isolates the line from the output.