how do I get ch to start heating?

System:potterton kingfisher2; drayton digistat RF and potterton EP2000 controller connected via a Salamander box. digistat rf responds OK to incoming heat call from companion remote room stat. RF shows flashing red when call for heat is received, then constant green. EP2000 time set OK, CH slider is set to 24H, but boiler doesn't fire up. It fires up OK for water heating, though.
Recently, CH began operating erratically, usually I got it going by operating the digistat setting down then up.
How to fix the problem, please?