Nobody really knew that Marvin and Welch could sing. Yes, The Shadows did backing vocals for Cliff Richard from time to time but most audiences assumed that, because they specialised in instrumentals, there wasn’t a strong vocalist in the band. It turned out that not only could Marvin and Welch both sing but that they were both perfectly capable of doing lead and harmony vocals, to considerable effect.
I can't find it on YouTube at the moment (I fully expected to find it) but follow this link to a well-known live Shads performance and listen to the way Hank and (specifically) Bruce effortlessly share lead vocal duties:
https://www.dailymotion.com/video/x2xomll
Bruce shifts into harmony so easily, and you don't even realise he's done it. Yes, it's mixed well, but hell, the man is a master. They might not have been happy with the recorded results of their harmony singing when they were young and just starting out, but thirty years of experience (not least the time spent with Farrar) meant The Geordie Boys became harmony singers who could contend with the world's absolute best.