Hi Jonty......welcome to TFF.
Your rifle appears to be a British Enfield Model 1858 two-band percussion rifle in caliber .577.
See here: http://www.edsmart.com/jz/guns.htm
(scroll down to "1858 Enfield Rifle)
These rifles would, indeed, been used by both the British East India Company and the British Army in India.
As for value.....sorry, I can't help you there.