Though it is highly unlikely that there was another Sparling in the regiment in 1853 who would not be present in 1854, it is a possibility. The association with Fenton makes it almost conclusive, but I agree that all avenues should be checked.
If you live anywhere near the National Archives in London, it would be short work to check the Muster Rolls of the 4th Light Dragoons for the year 1853 for anyone named Sparling.If not, I suggest you join Yahoo Groups Crimean War Discussion group and throw out the question there . .. there are several members who are very knowledgable about photography of the period and others may have 4th Light Dragoon records for the period in question . . .
Last resort, hire a professional researcher who does work at the National Archives . . .