Did someone else originally perform that theme?
Yes. I can 'just about' remember the Ed Stewart hosted era, and Chas n Dave theme tune was absolutely not in use then.
If I remember, back in the late 70's I think it was a jaunty orchestral tune, probably one of Ronnie Hazlehursts compositions, and the CnD theme tune (which wrongly, is assumed to be the theme all the way through the shows original run) was totally different to what went before it.
Nothing on you tube though - despite the fact that in one video, some idiot has used the CnD theme tune over the a photo of the badge from the mid 1970's era of the show.
My user name might look like Hatton Cross, but it's pronounced Throatwobbler Mangrove.