My apologies if this is an elementary bit of html that everybody bar me already knew!
I nicked this style of coding from a "proper" epub and applied it to a text I was working up. It appears to solve the problem of linking from a footnote citation in the main text to the footnote text and then linking back to the citation again.
The epub was created in Sigil (bless you Valloric!), and loaded to my Sony 505 via Calibre. The links appear to work in Sigil, Calibre & on the reader.
Example: - I've separated the significant bits to make them more obvious.
In file ..Text/Section0006.xhtml (an element of the main text):
<p>The five novels of this series cover only the earlier half of Judge Dee’s career, when he was serving as magistrate in various districts in the provinces. About this phase the Chinese historical records have little to say beyond the fact that he solved a great number of mysterious crimes. Concerning Judge Dee’s career at Court, however, those records go into considerable detail, for then the judge became a figure of national importance. He was one of the very few statesmen who could bring some influence to bear on Empress Wu, the cruel and dissolute but extremely capable woman who for fifty years ruled the T’ang Empire with an iron hand.
[Footnote <sup><a href="../Text/Section0035.xhtml#fnote1" id="fcite1">1</a></sup> ]
How Judge Dee tried to reform a corrupt administration and, falsely accused, was sentenced to be tortured to death; by what stratagem he succeeded in escaping from prison and how he effected his return to power; how thereafter he prevented the Empress from committing many a cruel deed and how, as crowning achievement of his career, he thwarted her scheme to place an unrightful heir on the Dragon Throne - all this proves clearly that fact is indeed stranger than fiction.</p>
In file ..Text/Section0035.xhtml (Where the notes come from) (no I'm not Charlie's Aunt from Brazil; err, sorry!):
<p>
<a href="../Text/Section0006.xhtml#fcite1" id="fnote1">[Footnote 1]</a>
The second part of Judge Dee’s career has been vividly described by Lin Yutang in his recent historical novel <i>Lady Wu, a True Story</i> (W. Heinemann Ltd., London 1959, Chapter 37); there his name is transcribed Di Renjiay.</p>
Hope this is of help to others!