This R40 tax form should be used to claim tax back. Consider it as a short version of the full SA100 tax form. However it is a strange form because a non UK resident cannot use it and often HMRC return this form and ask you to complete the SA100 instead. There is an R43 for non residents which again is only sometimes accepted.
The current official location for the form(s) is below:
Before completing either of these forms you should contact your tax office to explain your tax position and find out if they will accept this form from you. If you want to know how to complete form R40 let me know.