Thanks again, chef. That tip with DVDInfoPro worked. I formatted there, then the disc was written to by DVDA OK. I still don't understand why I need to do this now and not before, but such is life, especially under Microsoft.
Just to answer the other questions:
The dvd300i is the designation for the internal version of the 300 unit and this comes in two versions 300n and 300c. I recently updated to the latest 300n firmware (3.2) which is quite recent (end of 2004), but this didn't solve the problem. I'm using x4 media.
Apart from this problem, now solved for me with the pre-formatting, I am very pleased with the 300n, having burned piles of discs with never a coaster. However, since I now have dual-layer capability with the V3 of DVDA, I'll be looking to change to a DL-capable burner in the near future.
Once again, thanks to all. This is a great and useful forum.