Hi,
This could be his army records from the National Archives ref; WO97/2999/155
William HART
Parish of birth: Kew
Near the town of Richmond, Surrey
Age: 22 years 10 months
Gardener
Not married
No children
Attested to serve in the Fourth Regiment of the Dragoon Guards for 12 years
Previously in the 1st Royal Regt of Dragoons
Height: 5ft 10in
Complexion: Fresh
Eyes: Hazel
Hair: Brown
Any distinctive marks: None (Slight Varicose Veins of left leg)
Dated at: Newcastle Under Lyme
26 April 1867
Military History Sheet
Next of Kin: Charlotte HART
Married to Charlotte WILKINSON
St Pauls Parish, Dublin, 1st May 1876
There some more info on these pages if you PM me an email address I will send them to you.
I think this makes his birthdate around June 1844, no luck finding him elsewhere though.
Regards
Daisy