Playing along with the exercise (by temporarily ignoring everything else that indicates the Java is far more popular than C#) and just using your job site approach, it seems that the UK is pretty different from the US.

Looking at US job search sites, Java has about 2 times as many results as C# in the US overall, and 3-6 times as many in SF and New York. Java even has almost twice as many results in Seattle.

From indeed.com:

Java - 81,818 [1]; C# - 35,623 [2];

[1] http://www.indeed.com/jobs?q=java&l=

[2] http://www.indeed.com/jobs?q=C%23&l=

comments:

- Java has more results than even ".NET", which encompasses more than just C#: 73,971 http://www.indeed.com/jobs?q=.NET&l=

- C# has about the same number of results as Python: 35,072 http://www.indeed.com/jobs?q=python&l=

From StackOverflow:

Java - 794 [1]; C# - 479 [2]

[1] http://careers.stackoverflow.com/jobs?searchTerm=java

[2] http://careers.stackoverflow.com/jobs?searchTerm=C%23

From indeed.co.uk (the only one that has a similar ratio as itjobswatch.co.uk):

Java - 25,478 [1]; C# - 28,037 [2]

[1] http://www.indeed.co.uk/jobs?q=java&l=

[2] http://www.indeed.co.uk/jobs?q=C%23&l=

From indeed.com, Bay Area:

Java - 5,997 [1]; C# - 1,013 [2]

[1] http://www.indeed.com/jobs?q=java&l=San+Francisco%2C+CA&radi...

[2] http://www.indeed.com/jobs?q=C%23&l=San+Francisco%2C+CA&radi...

From indeed.com, New York:

Java - 6,941 [1]; C# - 2,548 [2]

[1] http://www.indeed.com/jobs?q=java&l=New+York%2C+NY&radius=25

[2] hhttp://www.indeed.com/jobs?q=C%23&l=New+York%2C+NY&radius=25

From indeed.com, Seattle:

Java - 3,832 [1]; C# - 2,124 [2]

[1] http://www.indeed.com/jobs?q=java&l=Seattle%2C+WA

[2] http://www.indeed.com/jobs?q=C%23&l=Seattle%2C+WA