Sharon, Peepal Tree has an excellent reputation as far as I know. When I took my MA, the course leader and resident Professor was E A Markham (a wonderful poet who is better known as Archie--I think I emailed you regarding him once or twice but could be wrong), and he published a novel with them, I think, called Marking Time. Archie is, despite his outward persona of the vague, creative intellectual, sharp as a tack and very knowledgeable about writing and publishing. If PT is good enough for him then I'm sure it's worth considering.
Incidentally, have you discussed this project with your agent? If not then I would. She can only say no if she's not interested. But it does sound like a huge project to me, with great potential, that much larger publishers than PT would be interested in.