Hi Helen
My guess is c1880-90. Cavalry uniforms stayed the same for long periods. However if you compare to the links provided by Neil you can see that yours lacks the vertical piping on the pill box. Someone with dress regulations from this period may be able to tell you when this change was made. Otherwise you could try their museum -
http://www.qrlassociation.co.uk/m_history_17.htmKen