I afraid I'm not much help but I would imagine that your leeks running to seed is just the result of a dry summer. Musselburgh is an old reliable variety. Perhaps look for a modern variety which has some resistance to rust. The other problem down south is the emergence of the dreaded leek moth.

I thought Musselborough was intended to be overwintered. I have always sown it in Spring an often planted it out in the ground vacated by early potatoes. Harvested it around Feb - March and it has only gone to seed it I left one or two in the ground too long in late Spring. Planted out so late it is unlikely to go to seed.
Agree with Jimmy, if you want a summer variety look for a different one.