I have a Baxi Combi 130 HE . When the CH is on demand it goes through the ignition cycle, ignites and then cuts out after about 3 seconds. It will then re-try after about 3 mins. and the whole process starts again. Sometimes it will ignite and run properly, other times it fails all together. DHW is fine and I have no fault indicators. I have tested the thermistors and boiler stat and they are fine. Could this be the actuator valve and if so, is it expensive to replace.