Servicing a faulty boiler

Heating would come on intermittently, morning or evening or not at all. I called a plumber to fix the problem and service the boiler. Heating worked for a few days then stopped again. Plummer called out again and switched it back on, 2 min job. Lasted a couple of days then Heating stopped working again, by now I noticed a leaky valve under the boiler so I had an emergency call out to get the isolation valve replaced. Paid my bill then heating went off again, called plumber who replaced a faulty actuator on zone valve which was the problem all along, plumber said he had no way of knowing only that it started to make a bit of a noise.everything works now. My question is, should this have been picked up sooner and should I dispute the latest bill to replace the valve? Thanks in advance