Tim W wrote:
Did I read somewhere that vinegar makes an effective and safe weedkiller? I

You may well have done.
Find out where it was and never take any of their advice again.

bought a gallon of clear 'spiced non brewed condiment' today for £2 thinking
I would use it on the cobbles where the cats come through the cat flap or
the paths around the vegetable beds. Anybody want to warn me of the
unforseen?

It will have negligible effect apart from making the place reek of
acetic acid and you £2 worse off. I love people who do kitchen sink
chemistry thinking they are avoiding "chemicals". Some of the recipes on
the web could get you into serious trouble with relatively common
ingredients. This one is thankfully harmless and completely ineffective.

"Non-brewed condiment" is diluted industrially manufactured acetic acid
with attention paid to making sure there are no nasty heavy metals in it
(ie nominally fit for human consumption).

You chose the wrong condiment from the chippy. Table salt would be a
more effective weedkiller (although sodium chlorate would be better still).

Regards,
Martin Brown