Any question answered...

For cycling? bear in mind the gluteus/thigh extensor range are the main useful muscles for cycling. You'll need to build the glutes and hams as well as the quads.

As for sets/reps/load:

for conditioning/prep: 3-4 sets of 8-10 reps of 50-70% 1RM
for size: 2-4 sets of 10-15 reps of 80+% 1RM (usually till failure)
for strength: 3-5 sets of 3-6 reps of 80+% 1RM

^this is based on a cyclist's program, using weights for cycling specific training and not a body builder or power lifter, though the principles are similar, and is open to opinion and some leeway, especially with the load%.
Additionally there would be explosive exercises with jumps and reduced load for speed. (not really applicable to the leg extension)
Your mileage with different exercises may vary

1RM= 1 unit equivalent to the heaviest thing I can shift?

And why use a leg extension machine that works your legs in exactly the opposite direction to a pedalling action.

Use a leg press machine till comfortable with bodyweight presses then squatty squatty squat.

It does work the quads though, which are used in cycling. I also use the leg curl and the squat-sled thing.

Squats can be done in many different ways too. Also you need someone to spot you when you're doing squats

I don't have anyone to spot me, and the restrained bar (Smit machine?) is always in use, rather annoyingly

You shouldn't use a leg extension machine. Gym machines a work of the devil, forcing isolated unnatural movement. Free weights are the way to go. Do squats, they work your whole body and soul.

Get some training first to do it right.

I'll be changing gym soon- maybe the new one will have more of a free-weight area, the current one has not a great deal in that department (Brockwell Lido)

Or fuck the weights and try pistol squats :|

http://www.youtube.com/watch?v=TWvIu7s-vIM

I'll give these a go