I'm assuming the weeds are coming through the pointing in between the slabs??
If so, you've got the same problem I had - pointing starts to crack and then the weeds grow through??
I've got 2 small patios and one main one, so I tried some "joint it" on the 2 smaller patios last year, before I decide if to use it on my main patio. Let me tell you, I'll be using it on my main patio this year - it's dead easy to use and it sets rock hard - no weeds coming through at all.
All the info is here:
http://jointit.com/
....... I got mine off Ebay by the way :)