My "Bloomsbury Dictionary of Contemporary Slang" says "fool, idiot, buffoon" in the context that I had in mind.
It also mentions one of my favourite pastimes - "To prat about" - to behave stupidly or irresponsibly, to do things in a disorganized, messy way.
To be perfectly honest, I probably learned the word when used in the sentence "Don't be a prat Paynie"
I should add as well that no disrespect was intended to Mr. Pratt's fine Motor Spirit
