Decathlin do what looks to be a good value tarp set up: http://www.decathlon.co.uk/tarp-khaki-shelter-id_2363223.html
It even comes with poles.
It's not a bad tarp, but the supplied cords are too short, the pegs too sknny and short and the poles are damned heavy.
However, all of this can be fixed easily enough and it's cheap enough not to matter.
The tarp itself is quite light.
