Recommendations on reasonably priced yet tasty dinner cruises on the Thames? Late September if that makes a difference.
"Tasty" and Thames dinner cruises don't really go together I'm afraid.
I'd honestly just do the daytime river cruise on the Thames, instead. You can take boats from Westminster to Tower Bridge, or even further afield to Greenwich. This is a nice way to just relax and enjoy the view if you're going to either places. If you want to have dinner with a good view, I'd suggest the Duck and Waffle instead. It's atop the Heron Tower. The Sky Garden also has a restaurant as well as bar.