Hi to both of you ....
Firstly, and I expect that "Gunner" already knows this, but I strongly recommend that you have a close look at 'The Long Long Trail' website http://www.1914-1918.net/
- this also includes a search function and links to a specialist forum.
Secondly, for "Birdylady" there are only two Medal Index Cards for someone named Ernest James West who was in the R.G.A. Unfortunately neither of them include any notes on the reverse side to help identify the soldier's home address, and neither could I find a pension or service record for either of these men. I am no expert, but wonder if these are actually two separate cards for the same man, as one has an early 4 digit number, and is recorded as being in France from 26 Nov 1915
Ernest J West, Gunr, RGA # 1358
Received the 1915 Star
but there is no indication that he received the British War Medal and Victory Medal which everyone got
The other is Ernest J West, Gunr, RGA 352566, who received the BW & V medals.
I know that men in Territorial Army regiments started off with 4 digit numbers which were changed to 6 digit in 1917, but these were infantry regiments. I don't know if there was any renumbering of men in the R.G.A., though it seems there were some T.A. units in the Artillery - see http://www.1914-1918.net/hac.htm
Of course I may be way off track with the above, and there are, as I think you discovered, other men just "Ernest West" in the RGA.
Have you tried local archives in the Portsmouth area, and possibly newspaper reports - as a local boy your grandfather may well have been mentioned in the local newspapers. Maybe you could also provide us with some more information about his - date and place of birth, marriage - was he married during war time, were there children born to him and his wife during war time? Any of these records may carry his regimental number.
Good hunting! Sylvia