Thanks again for the reply!
Some good progress made - I had to go to town today, so picked up a spindle of Verbatim AZO BD-Rs from WH Smith - only £10.04, which I was very surprised, for 10 discs.
Did another burn of the ISO and verify and all went successfully - so it was down to suspect media! The discs that have been giving trouble are now in the bin where they belong!
I had cancelled the other discs after getting the ones today but have now reordered them as they are the non AZO type, so am interested to see how they perform.