Strangely most of the DVD's in question were burnt on a Pioneer burner (circa 2004) and read back on LG burners (made 2008-9). We had a box of new burners available at the time, and I connected 2 of them up to each of our 3 comps at work to get all the DVD's read as fast as possible and later shifted via the network onto a 1tb hard drive.
At this time, I found that the discs that wouldn't read, invariably would (took 10 minutes in some cases to do it but it worked) when tried on another of the LG units. There was no visible damage to the discs, they were mostly Verbatim brand.
It wasn't one particular LG drive that was "bad" either. Some discs seemed to "like" certain units more than others.
Cant do any other tests on the discs, they were smashed up and dumped being successfully read.