Sorry found the answer.
The scripts had been downloaded as part of the Packt "Hadoop Beginner's Guide"
They originally had the shebang set as:
#!/usr/bin/env ruby
but this had generated a file not found error for ruby itself. Checking the details of env showed it used the PATH variable to determine the location of ruby. The ruby exe was in /usr/bin and this was in the PATH. However, I amended this to:
#!/usr/bin/ruby
and this fixed the original file not found error but produced the error in the question above.
I finally tried to run the Ruby scripts themselves, at the console, and this gave the result:
[paul@lt001 bin]$ ./wcmapper.rb
bash: ./wcmapper.rb: /usr/bin/ruby^M: bad interpreter: No such file or directory
This seemed odd as the exe existed in the directory shown.
I then recreated the script files (by typing them in at the console. This fixed the problem (with the scripts running both at the console and in hadoop). My assumption is that the format of the files themselves (possibly the ^M) was at fault.
In summary it was the interpreter that the "file not found" error related to even tho' the file listed in the task log was the script file itself.
Hope that helps some one.
P