I visited the Guyana National Archives situated in Main Street, during a visit in 2001, but was disappointed by the unhelpful attitude of the staff. However on a subsequent visit in 2004, I had better luck and was very pleased to get from them a photocopied list of population censuses for BG.
There were censuses for 1841 and other years covering the parishes, but I don't know whether the census returns were as we know them in the UK or simply a population count. I assume the former. I didn't have the perseverance to find out!
I understand the National Archives are now situated on Homestretch Avenue, but don't know whether they are fully up and running.
Perhaps someone in Georgetown can expand on this.