The photo was taken in late 1914, the soldier at the bottom left as the Imperial Service Badge on, this would make the photo around 1914/15, as he had agreed to served overseas after the War had started, The unit is a Territorial Force Yeomanry unit, as your man is from Essex, you could try the Essex Yeomanry first, 1/1st Essex Yeo 4/8/14 Colchester they landed in France 1st Dec 1914.
1/2nd Essex Yeo Formed Sept 1914, but never left the UK
1/3rd Essex Yeo Formed in 1915, but never left the UK