Potatoes disintegrate when boiled

I have recently harvested my second early Maris Peer potatoes. I had a mixture of sizes from tiny marble to good half fist size. I chose this variety because I often buy them from the supermarket and they are excellent as boiled potatoes.

The problem is that when I boil my home grown potatoes they just fall apart in the saucepan. I tried boiling on the same day as lifting and also boiling them after leaving them for 5 days or so after lifting. 

I bought the seed potatoes from my local, reputable garden centre and am reasonably sure that the are the variety they were marked up to be.

Any suggestions as to what I am doing wrong?