VailantTurbomax VUW242 E Central Heating Problem?

My Turbomax will, but only on occassion, fire up to send water around the central heating system. Domestic Hot Water is OK.New diaphram fitting 4 weeks ago on the Diverter Valve to sort a hot water problem but that ended up being the micro switch was faulty.System has Sentinel 400 in at moment and now awaiting to drain and add Sentinel 200 and 100. Any Heating Engineers in the know able to help me please?Thanks. John D.