Thanks for your reply,
I've re-downloaded all the files shown on http://bitbucket.org/bjoern/arduino_osc/src/14667490521f/libraries/Ethernet/
Errors now look like this:
C:\Users\Stuart\Downloads\arduino-0021\libraries\Ethernet\UdpBytewise.cpp:30:19: error: types.h: No such file or directory
In file included from C:\Users\Stuart\Downloads\arduino-0021\libraries\SPI/SPI.h:15,
from C:\Users\Stuart\Downloads\arduino-0021\libraries\Ethernet\utility/w5100.h:14,
from C:\Users\Stuart\Downloads\arduino-0021\libraries\Ethernet\UdpBytewise.cpp:31:
C:\Users\Stuart\Downloads\arduino-0021\hardware\arduino\cores\arduino/WProgram.h:17: error: declaration of C function 'uint16_t makeWord(byte, byte)' conflicts with
C:\Users\Stuart\Downloads\arduino-0021\hardware\arduino\cores\arduino/WProgram.h:16: error: previous declaration 'uint16_t makeWord(uint16_t)' here
C:\Users\Stuart\Downloads\arduino-0021\hardware\arduino\cores\arduino/WProgram.h:27: error: declaration of C function 'long int random(long int)' conflicts with
c:/users/stuart/downloads/arduino-0021/hardware/tools/avr/lib/gcc/../../avr/include/stdlib.h:504: error: previous declaration 'long int random()' here
C:\Users\Stuart\Downloads\arduino-0021\hardware\arduino\cores\arduino/WProgram.h:28: error: declaration of C function 'long int random(long int, long int)' conflicts with
C:\Users\Stuart\Downloads\arduino-0021\hardware\arduino\cores\arduino/WProgram.h:27: error: previous declaration 'long int random(long int)' here
C:\Users\Stuart\Downloads\arduino-0021\libraries\Ethernet\UdpBytewise.cpp: In member function 'void UdpBytewiseClass::begin(uint16_t)':
C:\Users\Stuart\Downloads\arduino-0021\libraries\Ethernet\UdpBytewise.cpp:46: error: 'Sn_MR_UDP' was not declared in this scope
C:\Users\Stuart\Downloads\arduino-0021\libraries\Ethernet\UdpBytewise.cpp: In member function 'int UdpBytewiseClass::available()':
C:\Users\Stuart\Downloads\arduino-0021\libraries\Ethernet\UdpBytewise.cpp:55: error: 'getSn_RX_RSR' was not declared in this scope
So still a little stumped, but learning fast..