Hey mate I had the exact same trouble on my old clio, but never fixed it I'm afraid. I think it has something to do with the cable that runs from that handle to the runner part of the seat, If you take the fabric off the seat you may be able to find the problem?
Can you still move it back and forth using handle under the seat?
What I ended up doing, was flipping the seat forward with the handle on the side, then moving it forward using the lever underneath.
Sorry I couldn't be more help.
Oh and one more heads up, it'd probably be cheaper to buy another seat from someone off here than it would be to get Renault to fix it.