Hmmmm,
Can't quite picture what you are trying to do....

I'm assuming that this is an open stairwell with a light fitting on the ceiling that dangle all the way to the first storey.
First off - hire a scaffolding/access tower - it's going to cost you about £100 but it's a lot cheaper than breaking your neck, remember if you fall from 2 storeys you will be falling 20ft+ any way you look at it it's going to hurt

.
Secondly (with the use of scaffolding tower

) once you get up there you will find that there 'should' be a ceiling joist in the vicinity - this can be used to secure the fixing for the light.
Thirdly - seriously consider getting an electrician (preferably one that is NICEIC registered and comes with a personal reccomendation from a friend or neighbour) to do this - this will probably cost you a couple of hundred quid but it will be worth it in the long term.
If you are still struggling (budget whatever) take some photos of the stairwell and the lamp, post them up here and I'll see if I can 'talk you through it'...
__________________