As a warning to anyone else who sets out to create a paperback with Createspace, be warned: if you get in a muddle with trim sizes (or whatever other problem) and decide to cut your losses, delete it all and start again, this is where the system clocks your ISBN and won't let you re-use it - not even for the same story. So don't delete - persevere, otherwise you will lose your ISBN.
Despite my 5 complaints and 5 responses from Amazon staff all assuring me I would now be able to re-use my ISBN, I could not; it still came up as Invalid in red text. Exasperated, I remembered they could only be bought from Nielsen in batches of ten, so I had a few still on file. I tested a different ISBN - with the trim sizes I wanted this time - and it worked. I have made progress at the cost of losing my original ISBN and considering they probably cost around £10 a time, that's quite a loss.
I suppose I can re-use the initial ISBN with another system such as Lulu, or Smashwords or whatever, It will be interesting to see if I can ever re-use it for another story on Amazon. All I can say is that Createspace has a definite slant on persuading people to use their own (free) Createspace ISBNs.
Another thing - the fifth and final response from Amazon told me I should be using Firefox instead of Internet Explorer 11. Evidently Createspace was designed to be used with Firefox because of "its encryption and security coding"- but why is that not made clear right at the point of starting out? There were other hints and tips such a reloading the page from the server, clearing out the cache on my pc so that it would work more efficiently, but if these things failed, then I should "contact my service engineer for a system revision." In other words - it was my fault.
I am using Mozilla Firefox for Createspace now and by using a new ISBN I have got the book to the proofing stage without any hitches this time. Hopefully when the proof copy arrives, it will be OK. We shall see!