I absolutely agree you need to worm again. And would also doubt the previous owner's assertion that they had no worm problem. In fact yellow wormers were well known for producing worm resistance 40 years ago - so in my book I would only use them if I'd tried everything else and still had a worm problem. In addition, the fact that your own field had been grazed by horse and cut for haylage in previous years would mean that it was more or less worm free when you put the sheep on it, so the only way your bought in sheep could realistically have had such a high worm burden is if they brought it with them.
I would now worm with Panacur, and I would treat them for 5 days . This should get rid of the rest of the worms, plus the tapeworms. Treat all 3 a half dose for 5 days.