You've probably found out more about your Marcus Sparling since your last posting to the list in 2004, but I have some information from Pay and Muster Rolls for the 4th Light Dragoons which might help you.
In the Muster Roll for the 2nd quarter of 1846, when the regiment was moving from Hounslow via Banbury, and then stationed at Manchester, M F Sparling was recorded as a private in the troop of Captain Alex Low. The subject of my research, Farrier George Clout, was also in this troop.
I've taken notes for Alex Low's troop for latter years, during their tour of Ireland, but Sparling doesn't appear, so he must have transferred to another troop.
In the 4th Quarter of 1852 Private M F Sparling appears in the regiment's band, along with George Clout's son James.
In the 4th Quarter of 1853 Sparling no longer appears in the list of band members. James Clout had been promoted to trumpeter, and his brother George Clout junior had joined the band in his stead.
Also, with reference to your discussion with Lawrence Crider back in 2004, the Major Halkett you mentioned was killed in the Charge at Balaklava.
I will be at kew later this year and will let you know if I find any other references to Sparling.