I have a rather large patio area and have just spent hours getting all the moss/weeds from inbetween all the slabs what is the best thing to put down to try and stop this happening in future, is there anything I can fill in between the slabs?

Cement!

Yes, but weeds can break through this as well. I suspect that the patio wasn't laid properly in the first place and hasn't got a weed suppressing membrane - or if it has this has deteriorated...

I would sweep gravel or sand into the gaps and then treat it on a regular basis with weedkiller... or lift and relay the patio...

Angela