If the head gasket goes, the engine oil and coolant/water mix. The engine will still run, but will overheat. Make sure you don't run it for too long or let the engine get too hot, because the head may warp.
If you take it to Renault, it'll be a fair few pennies to fix. The part itself is no more then £60, but the labour is the bestpart.
If I was you, I'd either, have a crack at it myself if you're good with a set of spanners. Make sure you buy a Haynes manual though. Or find an independant garage to do it for you. They'd probably only want £150/200 labour on it.
Might be worth getting the timing belt done at the same time if a garage do it....