I see Thompson and Morgan describe this variety as having 'firm, creamy flesh'.
Firm sounds ok, but 'creamy'..... um??
Try steaming the potatoes as this is a better way of cooking them anyway, but in future choose a truly waxy variety like one of the many salad potatoes: Belle de Fontenay is very good, if available in the UK, otherwise Ratte, Linzer Delicatess, or Pink Fir Apple. There are many modern varieties which won't go to mush.

Assuming you are leaving the skin on - if you are going to boil them then do not have water boiling fiercly - just a rolling boil and watch they don't stay in water too long- this way you might stop them going mushy. Or sprinkle with olive oil and cook as roasts.