Er the monoblock also looks a silvery colour where ive sprayed, will this fade or go away next time it rains?

I hate to tell you this, but there's a far easier way to deal with weeds growing in paving or paths. Go and buy yourself some Pathclear, or Bayer Path Treatment, mix it up as instructed on the packet, then water on with a can with a fine rose attached. Best done when the weeds are actively growing, usually late March early April, and on a day when no rain is forecast for at least 6 hours. These treatments have a residual effect, and should keep weeds away for the whole of the summer, though often by end of August, you may start to see a few appearing.
As for the silvery look, if it hasn't rained since you used it, it might disappear, but some products do stain stone/paving.

Normally mono block dont have cement joints ,the gaps between the blocks should be filled with kiln dried sand and if weeds are a problem here you can buy the kiln dried sand with a weed killer in it,or apply via watering can as mentioned.Also blasting with a pressure washer is a quick way to ruin the blocks.

Cheers guys

If i have stained monobloc is that is ruined! Worried now:-(

Been on a few websites some people say roundup leaves white powdery stain bu it fades away in a few days! I hope
so!