Ok so I am a little bored and decided to look some of this up for you!
It is "Cogswell & Harrison" - a very old gunmaker in London. I believe 168 Piccadilly was their address. There is a book written on the guns they produced. You can search for their name on the net to find it.
GRI stands for Georgious Rex, Imperator denoting the rifle was made during the British Indian Empire
RHI (are you sure it's not RFI?) RFI stands for Rifle Factory, Ishapore denoting the rifle was made after the partition of India in 1947. Could it be that the rifle was reserviced at that time or that piece is not original?
Try this website for most of the info I just gave you -
http://en.wikipedia.org/wiki/Lee-Enfield
The 1944 DI Z headstamp stands for Indian Government Ammunition Factory, Dum Dum, Calcutta, India. This was manufactured for the British and Indian army in India. The 'Z' refers to graphite glazed nitro-cellulose propellant.
This is from this website -
http://www.dave-cushman.net/shot/303headstamps.html
Maybe someone else can add to this for you