From "Do you remember?" online forum..
"... Alan Williams said Da Vinci was never in the Rubettes. 'The Rubettes' never existed when SBL was recorded, it was a demo recorded by session musicians and singers (inc Da Vinci) of a song meant for Showaddywaddy (hence the 'bop showaddywaddy' in the background)
But when they turned it down, composer and record company boss Wayne Bickerton still had confidence in the song, so remixed the track with the original Da Vinci vocal and released it with this random 50s sounding group name. When it looked like becoming a hit, it was a case of 'blimey, we need the original guys together to promote it on tv shows', they genuinely were making it up on the hoof. Da Vinci was signed as a solo artist to another company, and unavailable, so Williams was drafted in along with the original session band, and they then 'became' the Rubettes for promotional purposes.".