Thanks to ghost2, who found the crossover with Martha in for me. And, lo, here it is:
Alone in a Godless Universe (And Out of Shake’n’Vac) by Doyle
All Ages / Teen (Ish. Again, just for being Red Dwarf).
Word Count: 2405
Characters: Martha Jones, Dave Lister, Arnold Rimmer, Holly, the Cat, Tenth Doctor.
Basically, you remember that Lister was left pregnant with twins at the end of S2? Well, this is absolutely what happened next, although Martha can’t get the Doctor to believe it. Very, very funny.