Thanks for all the hints. I've sussed the phrase now and will construct the Playfair which should help me with 6 down and 25 across as I know what the other 6-letter acrosses are.
I've finished now. I didn't think of R for river, doh.
12a is an anagram ("shifts") of the first 2 words and the other indicated letter.
15a's second letter is not W -you must have 7d wrong.
34a is an anagram of a 6-letter synonym of "sat".
drxx - I'm not convinced by the definition in 32ac either, although OED defines the solution as "The company of persons who meet at a council-table". Even so, 'Company' on its own seems way too vague.