Hi
You can't get convict records from 'Croydon'. To be a convict means you have been tried and sentenced for a serious crime which in previous times you would be transported for and by the 1860s you would be serving time in one of the government prisons. Either way you were in the custody of the 'government'. Government generated records are held at The National Archives. If this is an enquiry about someone who was transported to Australia, then many of those records have been microfilmed and can be obtained in Australian repositories. The Rootschat Australian board will be able to advise on those sources.
Depending on which court the trial occurred in - assize records are held at The National Archives since again the judges appointed to the assizes were government appointments. Quarter sessions had county judges so those records will be held at the Surrey History Centre as Croydon was in Surrey. Court records will be limited to the indictment - the offence, and basic details like age and occupation and sentence (the court and prison system were largely uninterested in the man in any genealogical sense).
The National Archives guide on
Tracing 19th and 20th Century Criminals, goes through the other possible records
http://www.nationalarchives.gov.uk/catalogue/RdLeaflet.asp?sLeafletID=120and
Transportation to Australia 1787-1868
http://www.nationalarchives.gov.uk/catalogue/rdleaflet.asp?sLeafletURL=http%3A%2F%2Fwww%2Enationalarchives%2Egov%2Euk%2Fcatalogue%2Fleaflets%2Fri2235%2Ehtm&lBack=-1Regards
Valda
