My mother has a Vokera 14SE dosmestic heating appliance,6 rads,2 working lovely and hot,4 not working. Have bled the rads to expel air,still didn't work.Boiler cuts out after 1min then reignites after 5 mins,is this normal? Pressure gauge reads 1.5 which is normal,The appliance has a lockout preset button which I have used to see if that helps,it hasn't,what should I check next,we've tried everthing,what have we missed out on?