Hi Jeremy
You've read the link to The National Archives guide I gave
Criminals, Tracing 19th and 20th Century
http://www.nationalarchives.gov.uk/catalogue/RdLeaflet.asp?sLeafletID=120another useful source is
David Hawkings book - Criminal Ancestors: A Guide to Historical Criminal Records in England and Wales
The links was to a guide. The vast proportion of the aound 100 miles of The National Archives records for the 1,000 years of documents they hold is not accessible online. You will either have to visit TNA or since the trial was at the Middlesex sessions (the quarter sessions) the LMA which will also hold the calendar of prisoners which is a listing of prisoners pre and post trial and is probably the source you need to check first since it will give brief information about the prisoner e.g. age and previous form and (post trial) at least intially which priosn he was sent to.
PCOM2 is Home Office and Prison Commission: Prisons Records, Series 1 Covering dates 1770-1951 of which 473 volumes of records, it is vast, contain some prison registers in volumes 1-441 (many prison registers are lost or held in county record offices) but without knowing which prison (and it may not be held in PCOM2 if it has survived at all) you can't know which volume of the records you want to search.
Have you any reason to believe your 3 x great grandfather had any connections with Middlesex since there may be other reasons you have not found him on the 1871 census? Prisoners were included in the censuses, though sometimes only with intisals. Have you found him on the 1881 census?
Regards
Valda