Pottreton Perf 24 - Central heating goes on even if I swictch to hot water ?

When I turn the switch right for hot water only, I get the boiler firing up and providing central heating and hot water. The same happens when I turn switch to left. I just cant have hot water only without the central heating. Otherwise the boiler is fine. An engineer came along and thinks the micro switch needs to be replaced and says all in his charges will cost £ 100 +vat.
I am no techy but logic says the knob switch is not doing what it is suppose to do at the right position???!!!
Anyone..please advise.