Well that's a good start. You know your book dates from 1965 or before. Have a look at the rear flap of the DJ. Does it have a list of other books in the series printed on it? If so, is your title the last one listed? (Occasionally you'll find this on the back cover of the DJ).
If so - or if it is the only title listed, or if there are no titles listed at all - there's a good chance that you've got a 1st edition and that your book dates from the date stated on the title page. Always assuming there is one. Of course, if the title you have happened to be the last one in the series, then of course it will always be the last one listed. So this guideline is no use.
For some other exceptions to this rule you might want to get hold of a copy of Steve Rudge's wonderful publication "Index of UK First Editions". Email him
Now take off the DJ and look at the boards of the book itself and click here