The Roamerdrive is easily the best overdrive option for both Series and Defender models. Gone are the days of whiney, crude Land Rover overdrives that constantly have shifting issues. The Roamerdrive is a modern design that boasts a 28% reduction in gearing. All the parts are available and of course they come with a manufacturers warranty too.
For fitting instructions please see the following links:
Defender DS Roamerdrive: http://roamerdrive.com/pdfs/dsinstall_all_Nov2016.pdf
Series SS Roamerdrive: http://roamerdrive.com/pdfs/ssinstall_all_Nov2016.pdf