I'd like to nominate Amanda Craig, I've read three of her books which I think were superb, The Golden Rule, The Lie of the Land and Hearts and Minds. Similar themes to Jonathan Coe, state of the nation type of novels, but better than his imo.
Linda Grant is another author who I think has turned out some great novels, in particular We Had It So Good, Upstairs at The Party and The Dark Circle which dealt with the unusual subject of TB isolation units post war.
Any books you have loved by authors that fail to reach the best seller charts?