[Lord of Angmar]

Would that story be classed as 'uncanonical'?(davem)

I would say that, to an extent, yes, this can be classed as 'uncanonical', in the sense that we can reasonably believe that J.R.R. Tolkien did not want Trotter, the Cottage of Lost Play, etc. to contribute to the make-up of a reader's view of Middle-earth. The fan fiction you speak of may be 'founded' in Professor Tolkien's writing, but I think the idea of having a 'foundation' in (some of) Tolkien's (posthumously published) writings should be separate from the notion of what is 'canon' in Tolkien's works.

So it would be preferable to give the storya thorough introduction: a preface explaining what Tolkienish era the story is based in.(mark12_30)

Here is another distinction we as Tolkienites might want to consider making: era vs. revision. The character Trotter may represent an 'era' of Tolkien's career, but he does not represent an era in the canonical history of Middle-earth; Tolkein revised The Lord of the Rings and edited out Trotter, likely because Tolkien did not want him to be considered a canonical figure in his works (which is why no stories containing Trotter, to my knowledge, were ever published by the Professor himself).