No temperatures on my vokera boiler, what should I set it to?

I just had a plumber in to replace a safety valve after it blew. He does not work with gas boilers but said my boiler is the problem. Its a vokera mynute 2 years old. There are no temperatures on the control panel just settings for cold, auto and hot. What should I set it too as the plumber said it was set too high?
Thank you