Hi Tillergirl
IMO The Five Bells is about average. The food is okay and the atmosphere okay. Acceptable (remember this is only my opinion).
The Cutter is a very nice place to eat. it has a restaurant which it is best to book for, or you can have bar meals (which you don't need to book for). The food quality is very good IMO. For the restaurant, if you are going between Thursday and Sunday, it is very advisable to book as it is very popular. The pub is in a lovely position right by the waters edge. There is another restaurant just a few yards along from it which serves good food too, although I've forgotten the name of it. In Ely, there are several other restaurants (including Indian and Chinese) within a short walk of the riverside.
One small thing, I would recommend if possible, getting to Ely by early afternoon if possible as the moorings are v. popular, particularly at weekends.
Have a lovely holiday.