Maris piper is an early maincrop, so should be ready about 15/16 weeks,it's advisable to cut the haulms off and leave the spuds in the ground for 2 weeks for the skins to set and then store as above.
Alternatively you can pick them without the 2 week wait, they can be prone to slug damage,but be careful not to damage the skins.
After storage it's a good idea to check them after a fortnight, some times a bad spud can get through and spoil the rest of them.
And enjoy the Charlottes I've been taking a few out of each tub for the last fortnight and it's only today when I've completely emptied one,mainly because the haulm has started to die back.