That's encouraging in a twisted sort of way. My leeks germinated well but the seedlings have been more or less at a standstill for weeks. I can't think of anything to do to get them going.

Perhaps its a bit early to find them for sale - maybe everybody's are the same. Best persevere with what you have until you can find some better ones? I'm going to speak sternly to them - it worked with a campanula I threatened with the bin if it didn't flower this year and its absolutely full of buds.

When were they sown ?

My exhibition leeks are about 3 foot tall. They were grown under lights. Sown January.

Normal garden leeks sown February are growing away well and planted out weeks ago. Grown normally, no lights.

Leeks should be an absolute doddle to grow from seed if you have a little bottom heat and a suitable compost. I'd suggest John Innes seed as you won't be able to get the composts I use as they are peat based professional.
All I do is get a 20 compartment rigid tray, fill with compost, put about 5 seeds in each compartment, and wait. They soon germinate, no need to thin, just let them grow for a couple of months. Then, untangle and transplant them into the same 20 cell packs, with I'd suggest Jacks Magic compost. That is becoming more and more available, it is made by Westlands, but hasn't got any of the rubbish.
For snoopdog, Shawbury garden centre sell it, not far from you, and I think they have leek plants for sale.
Later sown seeds as found in nurseries will be spindly, that's how leeks are.

I've never needed to use a heated propagator for them in the past - they germinated very easily. Its since planting out that they have begun to sulk, and I have done nothing differently to previous years. I am going to try threatening them as I said. will go out and do it now before I forget. Watch this space.