Speaking as a farmer, I don't think £20 is anywhere near enough. We'd charge £25 for a young lamb as a set-on, but that'd be a lamb whose mother is still rearing a lamb, or two, and who is surplus to our requirements.
If the ewe had a single, you've lost a year's production from her, with the possibility that she now gets mastitis, needing treatment, and then has to be sold as a cull.
If she has twins, so is still rearing one, then you've lost the income from the second lamb. Costs of rearing the second twin on its mother aren't high, so I'd be thinking around £50, at least, plus disposal costs.