I have a similar problem with diagnosing a weed that has appeared in my
garden this Summer, but not that I can recall in previous years.

It has green jagged leaves and appears very quickly. In a small patch I
have seen five different clumps of these leaves.

If I dig one up, I see a single thin white root which I can never get to the
end of. This stuff has already started to appear in other parts of the
garden about 30ft away.

Recently I have started to use Roundup on the leaves and hope this will
eventually get down to the roots and source.

Any ideas whether it is bindweed or some other super weed ?

Mike.