You need to fit a thermostatic mixer valve to the output of the calorifier.

I fitted one a couple of years ago and it has made a big difference to the water temperature. It also has the effect of making the hot water last longer as it is mixed with cold.

It looks like this when fitted

[attachment=0:8upr7yj7]Mixer Valve.PNG[/attachment:8upr7yj7]

Unfortunately the old link I had to the Cleghorn Waring site where I bought it no longer works and I can’t immediately find a new link.

If you do a Search on this Forum for “calorifier” or “mixer” there are a couple of threads on the subject that might help.