For a lot of homeowners, the mortgage is the single largest outgoing every month, and also the one that gets reviewed least often. Other bills get scrutinised, switched, and renegotiated on a fairly regular basis. Broadband providers, insurance policies, energy suppliers — most households treat these as things worth shopping around on when the renewal comes up. The mortgage, by contrast, tends to get set up once and then left alone, partly because the process of changing it feels daunting, and partly because the original deal felt like enough of an ordeal that nobody is in a hurry to revisit…