Gardeners - is there such a thing as this?

I'd probably brush in a dry mix, I've never heard of expanding foam specifically for gardens

WSS. If you're worried about the weeds coming through again, I'd brush in the dry mix, over-lay the slabs with weed suppressent matting and then cover with pea-shingle to hold it in place.

Assuming you're laying the decking over the existing slabs you can put the joists down then place weed suppressing membrane between that and the decking planks. That essentially makes it dark under the decking so nothing grows anyway. Our carpenter did this over a grass/weed/soil area and we've had nothing coming through the decking.

Thanks - we will do that for under the decking. We also have an area that's not going to be decked so need to think about that too.

I'm no gardener and want to do things as easy as possible ?

The dry mix will be pretty effective at keeping the weeds from growing between the cracks in any case. Even if some do poke their heads through, it probably won't be until next year and even then, they're pretty easy to pull out from between the slabs. If you get yourself an old knife to wiggle down the sides of the slabs, that'll help with easing them out.

If all that is too much like hard work (I'm a lazy gardener!) Just spray the beggers with Round-up or similar and that'll see 'em off quick smart. Just make sure you do it on a calm, dry day and make sure you don't accidentally spray anything you want to keep as Round-up will kill anything it touches.