Likely a reference to the Newmarket Racecourse in Newmarket, Suffolk. The racecourse opened in 1636 and in the mid-19th century had seven annual "meetings" (formal races).

In London Labour and the London Poor edition: 

Phase 1

Statement of a Returned Convict. (Volume 3): "We made most at the races. My mate and his wife told me that at the last Newmarket meeting we attended, 65l. was made, but they rowed in the same boat. I know they got a deal more. The 65l. was shared in three equal portions, but I had to maintain the horse and cart out of my own share."