It does seem it's in progress. According to TV Scoop who report that:
"It was being posited last night that Detective Chief Superintendent Jack Frost might at the very least become even frostier with his superiors in future series after creator Rodney Wingfield's nephew was wrongly arrested on suspicion shoplifting. Wingfield has made a formal complaint to Essex police about the way his nephew was treated and in a letter to his agent he states: "My current pro-police Frost book will be my last.""
Full article here.
The last 'Frost' book was 'Winter Frost' in 1999.