2006 Citroen C3 2006 c3 1.6l 5 speed manual, not selecting top gear, seems to engage 3rd

Hi, I have only recently purchased the Citroen 2 weeks ago and everything seemed great until today, all of a sudden its not selecting top gear and seems to engage 3rd,, then when re selecting 4th and accelerating i noticed a severe slip in traction, i believe this is the clutch slipping and needs to be replaced? but im wondering if this is what is causing the issue with top gear not engaging or am i looking at replacing the gearbox as well?
Cheers