Just to round it up. I thought I'd add that Thompson and Morgan did eventually get back to me on this. (I used the info from Amphibian
thanks )and a bit more. They said
"I can confirm that the 'Bloomsdale' and 'America' varieties of Spinach seeds are not the same. The 'America' seeds were incorrectly listed as 'Bloomsdale' seeds on our system. I would like to thank you for your email as it enabled us to discover this error and correct it accordingly."
Well, thanks for thanking me, but I do find this customer service quite odd. They didn't offer a refund at all.
I've asked for one.
In the meantime I bought some from elsewhere, plus more postage.