Forget buying crowns, which is a way for the suppliers to make lots of money. Instead buy a packet of good F1 asparagus seed. In my experience EVERY seed germinates if you sow it in a pot or tray. Prick each seedling on very carefully into its own pot, and keep them growing until they are reasonably large. I have often planted them out in their permanent positions the same year as sowing.
You need to take great care of them after that making sure they are not choked by weeds or drying out, but once they are established you will have your 'year old crown' the next year.
OK you may have to wait another year before you harvest, but it is FAR cheaper, absolutely reliable, and the crowns don't have that massive disruption of being dug up and transported which is why so many die.
Of course, I should have said that you should still prepare the bed where they will be growing with all the same care, digging in lots of well rotted compost or manure and making sure every root and shoot of any perennial weed is removed.
I've grown all my asparagus this way and have excellent results, even though we are on clay soil.

i grew all mine from seed (30 crowns). they are five years old and after the very wet spring and cool summer, thought I'd lost the lot. But about 5 have sent up ferns that are as thick as a match and about 2' tall. they weren't brill last year, So I intend to move them to a new permanent position. In large pots (one per crown) into my polyt. It's the only way forward for me at this altitude and moisture levels. I will be doing it in the next couple of weeks.

Thank you Bertiefox for such a full and helpful answer. I'll sow seeds and see if I have more luck.
I don't have a greenhouse/poly tunnel -yet!- so can't try your suggestion 2ndhand, but thank you for replying.