Normally any document from you insurer that mentions your NCB and your policy number will be acceptable so you should first have a look at what you have. Had you made a copy of the renewal quote before returning it, that would probably have been acceptable. The NCB will have been mentioned on the renewal quotes for this year and last year as well as on other documents perhaps.
It sounds like they are saying that £200 is owed because they "automatically" renewed the policy. You should tell them, in writing, the date and time of the call (it'll be on your phone bill) when you cancelled as well as details of your move and, particularly, details of why the car did not need insuring between March and now. If they believe that you have used the car between March and now, they will have a strong case for asking for £200 and they wont send out any paperwork until it has been paid. There is nothing wrong with involving the ombudsman if you wish.
However, if you have any documents from last year's renewal, it may be possible to use those as proof of NCB. Your previous insurer may even post something to you if you ask - it'll be a year older than the current renewal, of course. If you are moving overseas for more than 6 months, you will find it difficult to insure the car without reregistering it in whichever country you are in (sometimes called matriculation).