S3 Gearbox Problem.

The gearbox in my nippers series 3 jumps out of both first and second gear. I have not driven it yet as the engine is out, but am hoping its just a simple repair.

Any ideas what to look at first, will it need to come out. Seat box is out as we were going to replace it but I have since questioned the previous owner and the above fault is all thats wrong...

Would be nice to keep this box rather than fit the S2 that I could really do with for my own project.

Cheers

Lee

perhaps the detent spring on the side of g/b see pic, this is easy , otherwise worn gears/selector forks