Great suggestions everyone!
I'm on the verge of scrapping the idea and going for a more varied programme, which will probably contain some fugues anyway.
Programme perhaps being:
A short piece (5 mins) by Beethoven or Schubert
Franck: Prelude choral and fugue
Prokofiev: Sonata no 7
Or
Bach: BWV 911
Franck: Prelude choral and fugue
Prokofiev: Sonata no 3 / Scriabin: Sonata no 5
will ping this as able, but immediately David Diamond's super cool piano sonata No 1 came to mind before i finished reading the post. the work is dedicated to Rosalyn Turek (first lady of Bach and one the greatest interpreters of his music of all time). The first movement is a massive Andante, molto espressivo (tempo primo) - Allegro, molto appassionato (tempo secundo) - Fuga - C, almost 16 minutes long.
3rd movement is a double fugue w coda
Interesting, I don't know this at all. Are there any available recordings?
Bach Toccatas 
I like BWV 911 the best followed by a close BWV 914. BWV 911 has two fugues. BWV 914 has one.
However, the Toccatas are only 10-15 minutes each.
You can also do Beethoven's Variations and a Fugue Op.35. There are 15 variations. This piece is about 18-20 minutes long.
III. Allegramente - Fuga (Double) - Coda
that one's about 8 minutes long.
freagin awesome work, should be performed/programmed more, ...more David Diamond in general but this piece in particular (if you want to get a jiggy taste of his skills, listen to his 2nd symphony, wow, probably the greatest american 20th century symphony )
[/quote]
mmmmm Bach toccatas are the best!
That piece is insane

I generally do not care for Reger, but his Telemann variations blow me away.
Played by Bolet of course. Hamelin is at his boring worst with this piece.
Thal
I've had this piece on the cards before, but decided against it for some reason or another