Hello Waffler, I find salt works well in cracks in paths , driveways and areas where you dont want to grow anything. I've used road salt, no need for boiling water, as soon as it gets wet it will disolve and poison the area with sodium. Get the level high enough and nothing will grow there.
Best wishes for your marquee wedding. Obviously you want the place looking really good, but remeber your guests are coming to celebrate with you, not to scour the place looking for weeds. People see the big things - not the odd weed in the corner.

Other than salt, Pathclear should work for you. I find it expensive and not entirely effective. You can spray it on topgrowth which it will kill. You then have to scrape out the dead brown stuff. I find it just as easy to scrape out the green stuff. And it stains the slabs. I've never noticed it stopping new weeds germinating.

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