I got fed up with tapping these out on the web form every month for the index-linked certs.
See here:
http://www.nsandi.com/savings-index-linked-savings-certificates
under "Interest Calculator".
It was formerly a simple GET, but NS&I decided to make it more opaque recently. I suspect the dread hand of some kind of obfuscating web framework.
I reverse-engineered the form submission and ended up with this simple script. 
It seems like it could be extended to other products with the manipulation of productID. This is currently a TODO.

To run:

python nsandi.py

should provide the help you need.