Had the flue for the stove swept last week. Not a lot in it, apparently, even after 2.5 years. The sweep said it's the double-wall system - it keeps everything hot and moving, so you don't really get creosote.
The stove is a lot more effective than the CH; CH is nominally 12kW but takes 4 hours to raise the living room temperature from 14 deg C to 21 deg C, although once it's there I can turn the heating off and the temperature will only fall by about half a degree per hour (the house is well insulated). The 5kW stove got it from 13 deg C to 21 deg C in just over one hour this morning, with a reasonable warming of upstairs too. For rapid heating from a boiler, you need more and bigger radiators than most houses have space for. IIRC central heating is normally sized so it can maintain 19 deg C indoors when it's -1 deg C outdoors; not much spare capacity for actually raising the temperature from cold.