Jonathon,
Couple of thoughts. If you do any more organized charity rides like this again, they all have techs to help. The idea is they want your bikes functioning before you start out. They also have people out on the ride to help.
The second is you should learn a little something about making simple adjustments and fixes yourself. It's never good to not know how to do repairs when you're alone. Buy a manual at your LBS or read up on the internet. Park Tools has a good site as does Harris Cyclery (Sheldon Brown).
Perhaps Rowletts would have been more accomodating if you told them it was a quick and simple adjustment. The way you presented the problem might had lead them to think you are looking at a several hour tune-up. That's another reason to become more knowledgable.