G'day All, With Gaz's help , we opened up the original gear box out of the yellow beast. We found that the little roller had come of the fork arm which was consequently badly worn.We assumed that this was the cause of 2nd gear jumping out under load. Would it be best to replace it with a new one, or would a good used one do? Thanks in advance.

Main thing to look for is a bent selector fork and chipping on the hard anodizing on the selector drum or excessive wear on the drum.
To check if the selector fork is bent put the shaft through the fork and check that the fork face and shaft are EXACTLY at 90°. This can often be seen as bad wear patterns on the fork where it engages with the gear. Hard anodizing chipping is very obvious and so is wear on the selector drum. I have found some straight aluminium drums which usually are very worn and US.
From my experience with the constant mesh motorcycle type gearboxes when they start jumping out of gear the selector forks are bent from the first time but may not show for some time.

Thanks for that info Rick.I'll dohe 90deg. check tomorrow and also have another look at  the drum. I guess if there are serious signs of wear, the drum and the three forks will need to be replaced. Will let you know and post up some pics. Thanks again Tom