I love mash potato and when it is done well there is nothing nicer. The cream is a approximation, sometimes the potatoes have more water in them than others and so the amount of cream does vary.

Ingredients
1½ kg potatoes, I like Vivaldi but Maris Pipers or King Edwards will work, cut into even chunks 
75ml double cream, you might not need it all 
25g butter, cut into chunks 
Salt 
Pinch of white pepper 

Equipment Needed 
Potato masher 

Method
Put the potatoes into a large saucepan of cold water  and bring to the boil, reduce the heat to a simmer and cook for about 15 minutes or until tender. Transfer to a colander and drain well, then return to the pan and set over a very low heat, start to mash your potatoes and continue mashing until really smooth.  When you think it is smooth enough, mash some more, keeping the heat on the whole time. 

Put the butter into the potatoes and stir with a wooden spoon.  Start to put the cream in the potatoes, stirring with a wooden spoon, you might not need it all, just stop when you are happy with the consistency.  Check the seasoning and adapt if needed.