This is a common problem and you are quite right to have fitted a mixer thermostat. It is also the case that the typical plastic connectors and the actual piping used are not certified for the temperatures in the calorifier. This was pointed out to me by Whale themselves. There is a special pipe (WX1519B) which should be used to connect between the calorifier and the actual piping as well as their brass fitting products.
The membership association for owners of Dufour Yachts