Problem with the diverter valve - vaillant turbo max boiler?

My boiler is 13 years old but in good condition so I don't want to replace it. We've replaced the diverter valve twice, the last time being just over a year ago. The latest one's been intermittently faulty since last October (doesn't like it when we start using the heating then switch back to hot water only). Last repair cost £240, plumber says now it's the micro-switch so we've bought a new one of those for £8 which he'll fit for £60. Is this likely to work or are we throwing good money after bad?