2001 Vauxhall Astra Can not change gear after sort usage

Hi
we bought the car and drove it for 10 minutes
the gears became sticky and hard to change then started to stick completely, we could not get the car into any gear unless we stopped and waited a few minutes...then it worked again for another 5 to 10minutes ,, then it did it again .. and again...
the clutch seems ok, the gears seem ok it just suddenly becomes impossible to put the car into gear..no noise or grinding just seems like the gear paths are siezed or blocked..what could this be
thanks
steve