captured MPEG files generate error message "video sequence header is incorrect" when adding to TMPGEnc DVD Author

Hi

Ive have several MPEG files of captured VHS via the composite input on my
ATI AIW 8500DV.
I chose DVD for the capture quality.

I've just tried TMPGEnc DVD Author for the first time today and found that I
cannot add these mpeg files.

I get the error message "The video sequence header is incorrect."

Is there a way I can fix these mpeg files?

They playback OK on my PC and I can create videoCDs with Nero from the same
mpeg files.

The TMPGEnc help file says "A video sequence header is necessary for every
GOP in a standard DVD."

What is a GOP?

Thanks for any info

Ian