Use 14% vinegar undiluted - it’s the strength that makes it so effective.
Salt is unnecessary if using undiluted 14% vinegar bought from any brico. In fact, when I initially tried it with salt it blocked the sprayer. 14% vinegar, undiluted, no washing up liquid or salt is amazing at killing all the small weeds that grow in gravel or any hard-standing.
Which, as I said, is actually *more* expensive than other methods such as the flame gun, and not much cheaper than pelargonic.
I suppose the one benefit might be that acidifying the soil will dissuade re-growth a bit.
I don’t use it on weeds growing in soil, just the very big courtyard, so no worries about it acidifying the soil. I don’t know what pelargonic is but it doesn’t sound like something I wouldn’t want my dogs to encounter. Vinegar is entirely safe for them as it leaves no residue once it’s dry, and it definitely does kill the roots of small weeds so they don’t grow back. Because the more I use it, the fewer weeds there are to treat, I find vinegar to be an incredibly cheap way to keep the courtyard clear.
With ALL methods of killing plants please remember you are also killing all the fauna around and on those plants at the time you spray - worms / spiders / beetles / ants / slugs / snails / aphids/ ladybirds / wasps / bees / butterflies / moths / not to mention the millions of smaller creatures we do not see.
Also, there may be lasting damage to frogs and lizards as they cross the ground that’s been contaminated.
Wherever possible, please hand weed to do the minimum amount of damage to our biodiversity. Or, don’t weed at all. Let it grow and then mow back to a green sward. I have masses of violets in the gravel alongside our cottage - I wouldn’t dream of “weeding” them. Ditto the Scarlet Pimpernel that grows there - reminds me of one of my favourite books from my teens.
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Indeed - pelargonic acid might be the least toxic to these critters - it occurs naturally in plants of the Geranium and Pelargonium families.
Given the price of 14° vinegar it’s no more expensive either.
I have no wish to harm Nature, which is why I’ve asked the question and followed the replies.
My particularly difficult spot is around an old wooden door at the top of some stone steps.
Flowering weeds have somehow got their roots enmeshed with the stonework of the building, the wooden doorframe and the steps.
Ants have (for some reason) decided this is a wonderful playground and the result is an ant nest in every possible (and impossible) nook and cranny… which is dislodging the soil which was forming some sort of bond in the stonework. (Don’t ask me how/why… 'cos I’ve no idea about this, I just know what I’m seeing/finding)
I’ve handweeded and then teased at the roots with all sorts of inventive bits and bobs but it’s no good. I’ve got to kill them (and I don’t say that lightly).
Plus, whatever I use will kill the Ants which are there.
But, I’m resigned to that now… as I can see no other way of getting them to move away.
I need to use something which will stop the plants from growing just in that one small spot and will take great care to ensure no other part of the garden/surroundings will be touched.
I intend to apply “whatever” on a dry day so that will be nothing running down into the soil at the bottom of the steps.
I’ll update, after I’ve plucked up the courage to do the dreadful deed… 
I hate to say it but careful application of glyphosate is exactly what you need.
A pity it’s not legal.
Pelargonic will kill the foliage rapidly, but you will need to keep reapplying it to any new growth (eventually the existing plants will give up).
We are a chemical free zone… which is why I’m worrying/dithering about vinegar even…
Anything you apply, even manure is “chemicals” at the end of the day - and you just said that you want to apply something…
Vinegar is probably the least “chemical” solution, that or salt. Pelargonic is supposed to be “natural” but I suspect the stuff on sale is synthesised industrially.
I tried salt last year… it worked for a while, but then they sprang back up and the ants thumbed their noses at me. that’s why perhaps a mix of vinegar/salt might work…
but I’ll try vinegar on its own first.
If you do try the salt and/or vinegar solution, do add a very small amount of savon noir to the mix. It helps the other ingredients soak into the weeds leaves and stems, increasing it’s effectiveness.
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