Thanks for the replies. I have to apologise- I've found the problem.
On this occasion, I used DVD Decrypter first, which produced a large decrypted VIDEO_TS file. I then used DVD Shrink to compress it to DVD maximum size. When I pressed "Back-up" on DVD Shrink, it gave the option of a. iso file output.
However, I've just realised that this is a pull down box, with options, one being to "Save to hard disk". I remembered that this was the option I had always used in the past, and produces a compressed VIDEO_TS file for burning. Tried it, and eveything worked the way it should- a backup that will play on my domestic DVD player.
I lent a friend my laptop, on which the programme is mounted, over the past few weeks- I suspect that he may have been playing with the programme, and left the ".iso" option rather than "Hard Disk". I shall have a word with him!
Thanks for the replies anyway- they were very informative.
One question- whilst I can see the need to turn a .iso into a VIDEO_TS file, I've seen requests in forums by people wanting to turn a VIDEO_TS into a .iso file. What would be the reason for this? Does anyone know what they would want with the .iso file?