It is all dowen to the variety of potatoe that you bought, Gerry. I have no experience of Jersey Royal but I do know that King Edward is what is known a 'flowery' variety, ie if boiled too much it will break down. These varieties are good for mashing and for chips. We grow what are called 'waxy' varieties which are much firmer and good for boiling and baking.

The microwave works very well for me. Slice the potatoes, place in a microwaveable container with a sealed lid. Add a knob of butter, no water, cover tightly, microwave for 7 minutes. Keep it sealed and wait 2 minutes after it stops. Potatoes come out light and fluffy. Season to taste. You can either make mashed, or coat with vegetable oil, transfer to oven for chips. 425F 20 minutes. Also works for sweet potatoes, beets and other vegetables. I never boil vegetables. It ruins them.

This might be of interest:

http://www.theguardian.com/lifeandstyle/gardening-blog/2013/sep/07/ask-alys-potatoes

Anecdotally, some people say its better to cut the haulms and leave the potatoes in the ground for up to a fortnight, or lift and store somewhere dark and cool for that time to allow the skins to harden off a bit. Ultimately, though, its down to how much moisture was available as they grew and how erratic the supplies of water were. Add plenty of manure next time before planting (this helps with good water retention) and water regularly and deeply when necessary.

Jersey Royals shouldnt boil to mush though surely?

I guess it depends how long you keep it on the stove.

No, they shouldn't Stera - but they might if the DM content is too high because of irregular and insufficient water supplies.