My sister's partner just had this problem with the ECU on this type of Clio (same age too). He had the option of having it fixed very quickly (i.e. 1 day/2 days) which would have cost him close to £750 or more slowly, which took about 10 days, for £300 ish. Independent garage. He chose the cheaper option. Best bet is to ring around for some quotes.

This seems to be a common failure; it misfires on 2 cylinders or just won't start at all when the ECU goes kaputt. It needs software though, so I suppose you can buy the parts but more than likely you won't be able to remap it.