http://www.accidentalsmallholder.net/forum/index.php?topic=32047.msg321956#msg321956is the topic that Cosmo is referring to.
I posted on that one also, but wanted to add here that selenium may be better given in these lambs in an injectable form, either repeating a short acting one for a few days, or a long acting injection.
Sorry to hear your vet wasn't much help though! They should be happy to give you some selenium, and a bit of advice on how to give it if you're not sure.
It's always a good sign if everything else is OK, they're bright and eating, so well worth giving a shot.
I have seen the occasional neurological lamb as well, I can't actually remember whether or where I have posted on here, but I did see some texel cross lambs that tended to get floppy and fall over when excited, but with a bit of physiotherapy - getting them up and walking around they came right (although still occasionally fell over when excited as grown lambs ready for slaughter!)
I think if you get the recommended selenium in by injection, and check any other deficiencies common in the area with your vets, and a bit of TLC, they should come right.
All the best
Suzanne
PS just noticed your second post, it may or may not be white muscle disease/ selenium deficiency, but selenium isn't very toxic to sheep so giving them a dose won't hurt. The vet is quite right that copper has a fine balance in sheep, there's not a lot of difference between the amount that's deficiency, and the amount that's toxic to a sheep, so he's quite right not to inject if not sure about copper. I think the selenium and getting them up for a few days may well just sort them out, so

for you.