Opening a project created in an older version of the program (version 13) in a newer version of the program (version 15) can often lead to buggy behavior. So that could be complicating the issue. Although you say version 13 could not create the DVD either.
You don't say which operating system you're using, but Sierra has been giving people a lot of trouble. I don't think that's what's going on here though.
You are trying to squeeze a lot onto a single DVD. Most DVDs can old about 80 minutes of video. But if you have the Fit to Disk option checked, you should be okay.
What's interesting is that the operating is failing at the same spot every time. Do you have a gap on your timeline that doesn't have any video on it? That can cause transcoding fails. So can having a Stop Marker at the end of your movie.
To see if the file size is an issue, try doing a Save As to save a copy of your project and then cut your project down to about 30 minutes. Are you able to produce an ISO file from this test project?
Thanks for taking the time to help, much appreciated. Answer to your questions
Yes - operating system is Sierra 10.12.2
I tried cutting it down to 30mins but again same problem. When burning to ISO crashes at 1% (forgot to mention that burning to disc always stops at 4%, burning to ISO 1%). Also tried burning to ISO at smallest size possible, again freezes while burn at 1%.
Interesting about what you said about something buggy near the start, there are no gaps and no stop marker, but i will trying deleting the first 30 mins and burning the second half to ISO and see if this works.
Any thoughts much appreciated
Deleting the first 30 mins didn't make any difference. I then tried deleting the Menu and tried again and had a successful burn to ISO!
So it appears the problem was with the menu. I'm going to remake it and try again...
The Project is quite long - 1hr 15mins - i'm using a 8.5GB disc
No its not long even 2 hours should fit on a single layer dvd.
Try a different menu.
(could you tell us which menu you were using).
I was using the General Slideshow Menu which may have been the problem.
Burn was successful using one of the cheesy wedding DVD menu's.
One of the reasons i upgraded from version 13 to 15 was i was hoping for something a bit more classy or just 'plain' in the DVD menu's but they were exactly the same. I don't know if any of the developers read these pages but could we please have something simpler or just not so naff in the options for menus? Just a background we can put our pictures on would be fine.