[Annen]

Did they miss the big freeze in winter then?

They were in module in the cold frame from October last year and were planted out in the bed 26th December.  I'm assuming they got frozen sometime.  Is that a reason for cloves not forming?

[DD.]

Yes, garlic needs a cold snap in order to clove. A good reason for not mollycoddling them!

[AnnieB]

How does Spanish garlic we see in supermarkets get a good cold spell as I would have assumed Spain was not prone to frosts in the way we are ? 

[Annen]

I thnk I have solved the mystery of the uncloved garlic. The one I stripped back does have cloves after all, but under quite a few skins.  So I think I've pulled them too soon and they aren't quite ready.
Doesn't help with the topic of the original post, of course 

[arugula]

How does Spanish garlic we see in supermarkets get a good cold spell as I would have assumed Spain was not prone to frosts in the way we are ? 

Spanish garlic isn't supposed to grow in our climate and doesn't need such a cold spell, where there are varieties which thrive with a colder spell. Apart from good husbandry, this is another reason not to use supermarket garlic cloves for growing.