Why does my shower (connected to mixer tap) always have scalding hot water!

[Chocoholism]

It's a rented flat I'm in so fixtures all in place, the bath comes with mixer tap and shower hose attached to it with a knob that you pull to change from tap to shower but even though mixer tap water could be fine temp with both taps running the shower will still output scalding hot
Any ideas what's going on here and anything I can do?

Call the landlord! Seriously...that's his/her job.

I had that for a while. just means the bit inside the shower that is meant to limit hot water flow is broken. as pp said, call the landlord!