Leek forming seed heads

I've been growing leek for as long as I can remember. Always same variety (Musselburgh) and same routine. Start off in heated propagator in April and plant out in June into well manured soil with a sprinkling of Growmore and pasture fertiliser every month.
For the first time ever in my patch, they are developing seed-heads in early August. (I'm just snapping of the seed-heads and leaving them in place.)
Couple of questions: Why are they seeding so soon? They normally don't seed until the following spring, and will they carry on growing in size despite seed-head removal?

The seed heads or 'scapes' are very tasty stir fried!