Hi, Neil,
Yes, what you show is as you say a St George set,
these sets were popular for club play in England from about 1800 to 1900.
But still on sale up to 1940, see Guy Lyons article, here.
http://picasaweb.google.co.uk/chessspy/GuySChessArticles
The set you show, is a late example(1920-1940) and would have cost about 2 shillings, due to the Staunton Pattern sets becoming into general use.
the set in question has a retail value of between £20 and £80 if in good condition.
There is a place in a chess set collection for such sets as they have a historical note.
yes the black side is probably ebony.
No don't clean it, as it looks to have been roughly cleaned already and has precious little genuine patina.
The difference in the carving of the knights may be due to the poor pay of the carvers at that time, or they may be replacements.