Thanks for replying. However after spending nearly all weekend trying to sort this out, I finally found the answer and it was so simple.
On the page where you make the video production, click on Tools at top of page then options.
The drop down has a heading at the top called "Render Using" and under that you have the choice to select either hardware or software graphics test.
Mine was set on Hardware but when I changed it to Software, the problem was cured immedietley.
Haven't got a clue what it meant but I had tried everything in desperation.
Many thanks again Frank for trying to help me.