spur on lighting circuit?

[LTPHI]

ive been left with the job of finishing off the electrics my dad started before he died. the main job is wiring a switch light into the loft, and i was wondering if it is safe to put it on a spur off the lighting ring thats already there(as there are no spare fuses). if this is possible, could someone explain how to do it please?

[Paper Spark]

Try here, it explains all
http://www.diynot.com/forums/viewtopic.php?t=7553&postdays=0&postorder=asc&start=20