Tomb in Space is the payoff story from Perfect Day. Here, at last, Julia gets to use the information that The Doctor told to her in the park. The TARDIS is damaged through Chrístõ’s plasma storm surfing and he has one chance – to key in the co-ordinate that Julia has memorised

&#x3A8 IS hexadecimal for Psi. At least according to the list I found online. If not, who knows. Apart from programming the TARDIS in machine code, what would that be used for? I can’t quite imagine. But it looked good.

As for the destination – the tomb in space – the TARDIS had to be in default mode, of course. That had to be so because Chrístõ already has an idea that his future incarnations will have a TARDIS in police box shape. At first he doesn’t know that the tomb is his own. It is only when he sees the sarcophagus that he knows. Not only because of the name, but the inscription which refers to one of his own favourite Earth songs.

The Impossible Dream, in fact, could have been written for Chrístõ or his later incarnations. The words resonate with his kind of courage and effort. Of course, it is from Man of La Mancha, the musical about Don Quixote, a man who fought impossible odds just as The Doctor did, though with less success.

Of course, for the older Doctor to tell Julia the co-ordinate so that he would find his own tomb at the end of time is a recursion that should be impossible in anything but science fiction. But what the heck. This IS science fiction.

To dream ... the impossible dream ...
To fight ... the unbeatable foe ...
To bear ... with unbearable sorrow ...
To run ... where the brave dare not go ...
To right ... the unrightable wrong ...
To love ... pure and chaste from afar ...
To try ... when your arms are too weary ...
To reach ... the unreachable star ...

This is my quest, to follow that star ...
No matter how hopeless, no matter how far ...
To fight for the right, without question or pause ...
To be willing to march into Hell, for a Heavenly cause ...

And I know if I'll only be true, to this glorious quest,
That my heart will lie will lie peaceful and calm,
when I'm laid to my rest ...
And the world will be better for this:
That one man, scorned and covered with scars,
Still strove, with his last ounce of courage,
To reach ... the unreachable star ...

http://htmlhelp.com/reference/html40/entities/symbols.html
http://en.wikipedia.org/wiki/Man_of_La_Mancha