Just because it was serviced doesn't mean they have to fix the leak. If it was a good enough garage they should of produced you with a visual health check sheet. Any raised concern's should of been noted and then passed to you for authorisation to carry out the work. Ive seen a good few clio's also suffer from rear main oil seals, check on the undertray for any signs. It can be easily confused with weeping gearbox's though! Give R Sport a shout and get it booked in, there down london way and are good with renaultsport's!!