My most common problem when doing plumbing repairs... 'Is turning the water off'...
If access was possible, the plumber could have froze the pipe while he fitted a new stopcock.
...Why would your water company send someone out to remove an ants nest..?
If your stopcock in the street has a nest, I'd have had a go at smashing the head of the stop cock key through it. They are usually very dry and break up easily.
Give it a few minutes while the ants go berzerk.
Then wrap a bin bag round your arm and scoop the remains out.
If that stop cock happens to be seized... then and only then... will your water company get involved.