There is a unpublished  manuscript by  R. Proud, on the title “On centralizers of unipotent elements in algebraic groups”.
If some one have a copy of this please post it. 





 (This unpublished  manuscript is used as a reference in the following papers:
"Centres of centralizers of unipotent elements in simple algebraic groups" by R. Lawther and D.M. Testerman )