Don't know if you read my post about lame lambs but sounds very like your one and vet is adamant that it is joint ill despite there being no swelling. He prescribed Betamox, 1ml for 4 days. Like you mine are all iodined, and they're given Spectam at birth but it's been a big problem this year. Upside is that they do get better and although I've had lots of cases none have died so should be okay!