This is how we make our triple cooked chips where I work, we use agria potatoes but maris piper should work just fine.
Peel, cut and soak in cold water for an hour. Give them a good wash then drain. We steam ours but boiling in just as effective, you want them just barely cooked through. Strain and leave to air dry 20-30 mins.
Blanch in 140c oil for 3-5 mins. Remove from oil, drain, place on a tray lined with parchment paper. If you're doing a lot add layers of parchment between batches.
Freeze them - This further reduces moisture and when you fry them to serve you can use a higher temp and fry for a little longer, means a crispier outside with a nice fluffy inside.
The advantage of this is you can make loads, freeze them and store them basically forever.
When you're ready to serve you fry them again in 185-190c oil for 4-6 mins, season straight out of the oil.