It will be the rubber bush link between rod from gearbox and rod from gear lever. Look down between battery, brake resevoir, and throttle body area and you should see the bush in question. When it fails it still alows fore/aft movement (3rd and 4th OK) and rotation in the direction of rubber compression (5th OK), but when rotated so that rubber is in tension thats when the split rubber shows up and you loose 1st and 2nd. Temporary fix is to wrap wire round it. Easy to change just two nuts and bolts if I remember, but heps if you have long arms!