Council tenants who have been overcharged rent by Cambridge City Council for up to two decades have received letters about the mistake. But the affected tenants will not find out until the summer how much they have are due to receive back as a refund. The council has identified an error dating back to when the government told local authorities in April 2002 to show service charges – such as for cleaning in communal areas or grounds maintenance – separately from rent, to show how housing costs were being calculated. From April 2004, the council included gas maintenance and electrical and mechan…