Should i get it fixed? how urgent is it?

my diverter valve on my combination boiler is acting up. how long should it last before it becomes URGENT. i cant get hot water all the time, but sometimes, and i REALLY hate letting unknown workmen in, cuz of past BAD experiences with them, how long before a faulty valve completely packs up? can it do further damage to the boiler? when i turn on the hot water and it doesnt work the boiler automatically cuts out so i get no heat either so it doesnt overheat. i know it would be ideal to get it fixed but its really stressing me out about letting unknown workmen in. i tried to find a place that has women only plumbers but i cant. not sure if my gas company employs any women plumbers in my area. ive been pencilled in to have it looked at thursday, but im thinking of canceling, would this be a mistake?