The position of the various letters matter. The explanation for the SY/Shadow series cars is pretty far down the page.
Specifically to yours:
S, in first position = Standard Steel Saloon (i.e., not coachbuilt)
R, in second position = Rolls-Royce (as opposed to 'B', Bentley)
G = 1978
and 35259 means that it was the thirty-five-thousand-259th car produced in the entire series (that includes RR and Bentley).
There is a plate that is generally attached to the underside of the left side of the hood/bonnet that will have your paint & trim codes. You can also order your build sheets from the RREC in the UK and all that information, and plenty more, will be available there.