Currently with Chris Knott too. I am changing from a Kia Pro Ceed GT to a GTi. After calling them they advise all I need to do is make up the difference for the remaining months. Extra £30 over the last 4 months.
They are very good too at comparisons, even after someone driving into my car and them accepting liability they didn't factor in the fact i made a claim and my renewal was about the same as the previous year bar a few quid.
Also add parents to insurance, mine reduced by quite a bit, even though they never drive the vehicle.
Comparisons to get a baseline. Add parents as named drivers.
Then contact Chris Knott. They've helped me when Flux wanted another £1100 (I'd already paid £400 for the previous car) to insure my GTI when changing 3 months in to years insurance. Even beat my cheapest comparison quote by £20.
Pay in full if you can.