I use Maris Piper with butter, milk and seasoning, but I'm wondering if there is a better variety for mashing?
The variety of potato used for mashing really is a matter of preference, and the style of product you desire.
Light, Fluffy Mashed Potatoes
The best potatoes for light, fluffy, very smooth and elegent mashed potatoes are starchy varieties, such as the US Idaho.
Dense, Creamy Mashed Potatoes
For dense, creamy mashed potatoes (which also hold longer), a waxy potato like a Red Bliss or a fingerling is more ideal.
Middle of the Road
You can also get excellent results with middle of the road potatoes which are between the starchy and waxy extremes, such as the US Yukon Gold.