Hello chrisjo:
Was the 5 minute render doing the radiosity solution then rendering the solution entirely from scratch, or was the solution already done (in the viewport cache)?
Are you working from a copy of their file? Maybe they set particular objects to override radiosity to not emit and/or not receive? This could affect the solution time a lot.
The number of triangle is almost always the determining factor in how long the radiosity solution will take. The other is how uniform or localized the direct lighting is. When you press Start in the Custom Radiosity Options dialog, how many emitting and receiving triangles are listed? As a guideline, I try to keep the number of receiving triangles below 100K, otherwise things move too slow for my patience level.