How can I make my Baxi Solo 3pf 80 cycle shorter?

My boiler is suffering from an extremely long cycle (20 min. on / 60 min. off). The thermistor cuts out at 80ºC flow as it should but the boiler doesn't re-start until ~22ºC.
By this time all the rads are cool again and I can't get the house hot enough to work off the room thermostat.
I've fitted a new PCB a month ago as it was burnt out, it came with a new potentiometer fitted and I fitted a new thermistor.
Any ideas welcome?