A charming alternative to ‘dull as ditchwater’. I also like the last affirmative sentence.
“Your brother is the most interesting patient I ever had. My patients mostly are as dull as suet puddings … But your brother is something different altogether, and I’m proud to have plastered his scalp.”
Source: Arthur Ransome, Coot Club (London: Jonathan Cape, 2009 (1934)), p. 118