I think there were some decent debt collection agencies listed on another thread here - i dont have it.
Thats one option if you want to go that way.
Another is a Solicitors Letter, allthough if revenue are chasing them they probably arnt going to be unduly concerned about that.
Id say start with the largest balance, if you feel comfortable doing so call over and politely ask can you collect what your owed, state how long your owed and ask can it be resolved, etc - to the individual/business as people find it more difficult if your standing in front of when refusing to pay over the phone/ ignoring letters emails and work from there.