Any experience of recruiting ex-offenders?

Hi, I work for a Hospitality business in Greater London. We've been exploring different avenues of recruitment and areas where people might fall into a socially marginalised group. 

One area that piqued my interest was Ex-Offenders. We have a HMP with a YOI wing not far from where we are based and I've seen organisations like "The Social Pantry" who state a percentage of their work force are made up of employees from an ex-offending background. I've been asked to gather a bit of information about this, but I'm a bit at a loss of where to start. 

Has any got any experience in this area or advice on what worked, what unexpected challenges did you face? 

Thanks in advance