The water is always too hot!?

We have only just had a Junkers WR-11 water heater fitted and have found that the water is too hot. I have increased the water flow and reduced the flame size, but to no avail! On the mixer tap in the shower the water stays hot and even though we increase the cold water flow the boiler cuts out. We have tried to get the engineer back, but it's now August and don't expect a reply anyday soon. Can anyone help??