Hi, Paula!
My wife and I rented a 26' narrowboat for a short break (3 nights) this past September. The Kennett &Avon Canal Boat Hire is a small, family-run business. They have 3 boats (we hired the smallest), the last time I looked, so they do not have their own marina. They have a mooring in Bathampton.
Richard and Sue Warren run the business. Richard provided instruction and went with us through the first lock out of Bathampton. He was willing to spend as much time with us as we needed. Truth be told, as the pilot I probably should have taken him up on the offer and worked through a few more procedures! Don't be afraid to ask Richard to go over anything in the training a second time. We found Richard to be friendly, supportive, and reputable. My seamanship will not be heralded in the pubs along the canal, but we had a grand time, all the same! Upon arrival back in Bathhampton, Richard was even nice enough to give us a lift back into Bath. In summary, we had no problems involved with renting or returning the boat. We'd have no problems working with them again.
Richard will advise you to make your major potty trips at pubs along the way. Very good advice. Even then, I also advise making at least one stop at an Elsan point, going and returning, to empty the cassette toilet. You definitely do not want to wait too long!
One final tip: It's not how far you go, it's how much you take the time to enjoy the trip!
George