Yes it should be R. purpurea, perhaps purple colour is confusing for Tanay. R. alpina has purple flowers and corolla tube much longer whereas both R. purpurea and R. capitata have mauve or light blue flowers and with shorter corolla tube. R. capitata has numerous flowers in the spike.
Dr. Gurcharan Singh
Retired Associate Professor
SGTB Khalsa College, University of Delhi, Delhi-110007
Res: 932 Anand Kunj, Vikas Puri, New Delhi-110018.
Phone: 011-25518297 Mob: 9810359089