Gerould was writing in 1957. A lot has changed since then.

The Viking influence, you are talking pre-conquest, Pseu. Any similarity between foreign ballads and those in English is down to translation by quite literate people. The whole of the north of England was under Viking rule with its capital at Jorvik (York) but that was a thousand years ago. I'm pretty certain at least some of my ancestors were Scandinavian.

I have a copy of Gerould but I haven't time to reread it just now. Plenty of artefacts and names have come down to us but you're asking a lot for any literature to have come down after a thousand years.

As late as the 19th century Danish fishermen landed their catches in Hull and it was said that local farmers could hold a conversation with them due to the local dialect of the East Riding containing lots of Danish words which had survived. But to the best of my knowledge no ballads changed hands.