You'd have to be having a very bad day to find yourself in this position! If ever you find yourself in the situation where the gear will not come up once you've lowered it make sure the runway is YOURS before you lower it i.e. it's sterile. That means ATC should not allow anyone else to T/O or Land on that runway, so even if you go around you still have a strip to come back to. Of course if you went around due to "nothing seen" then the question has to be asked as to why you were commiting to a questionable field in the first place.
If you do find yourself having to divert then Total Fuel Flow in Kgs/Hr divided into Fuel on Board should enhance situational awareness no end! Alternate runway centre fix as the "to waypoint", ETA on F-PLN/ND, F/Flow x minutes = fuel required...... will you/wont you? Simplisitic I admit, but that's how I'd play it if I ended up in such a dire situation - and I'd make sure the ALT was sterile long before I arrived as well.
A4