Why aren’t the potatoes cooked

It's almost certanly a variety/potato age thing.

There are thousands of varieties of potato and different countries grow different ones based on climate and local taste. You can group potatoes in to two sets:

• Thicker skinned potatoes that spend longer in the ground as they grow slowly. Varieties sold this way tend to be be a colder weather crop.
• The other sort are thin skinned, waxier varieties which include 'new' potatoes. Varieties suited to this grow faster and tend to grow better than the other sort in warmer weather.

There are 'waxy' firm varieties of the first group but most would be termed 'floury'. These are dryer potatoes and make great mash and chips and roasts and fluffy baked potatoes.

Waxy potatoes are better for holding together in dishes and salads. These just don't crisp up as well. I assume you're just getting these varieties.

Once the potatoes are overcooked, there is little you can do to give them structure again.

Your best bet would be to re-purpose the dish, making them into potato pancakes, a home fry, or some other type of dish, depending on how much structure is left.

In the future, you can minimize the chances of mushy breakdown by:

• Use a waxy potato (such as the US varieties Red Bliss or fingerling). These hold their structure better than starchy potatoes like Russets.
• Start the par-cook in cold water, and remove the potatoes as soon as it is comes to the boil. This will still shorten your roasting time, but minimize the likelihood of severely overcooking.
• Add a small amount of vinegar to the par-cooking liquid, which will help the pectin retain structure.
• Depending on the variety of potato you are cooking, don't cut them up into small chunks. It is much easier to overcook small chunks than larger ones.
• Don't par-cook. You will require longer for roasting, but it is slower and more controlled, and less likely to overcook.