The scrap man almost certainly got it for nothing but that don't mean it was nicked. We have a scap van come down our street every week collecting stuff people want to get rid of as well as old fridges and other white goods he seems to pick up a lot of old bikes. A quick £30 profit probably seemed good for him as I suspect most people would think a steel trike frame would be of little value and its scrap value will be well below £30.
Sounds like an interesting project, buy it