Do Trevor Horn and Keith Relf both play harmonica?
no
Trevor Horn
Trevor Charles Horn, CBE (born 15 July 1949) is an English producer, songwriter, musician and singer.
 His influence on 1980s popular music was such that he has been called "The Man Who Invented the Eighties".
Keith Relf
William Keith Relf (22 March 1943 – 14 May 1976) was an English musician, best known as the lead vocalist and harmonica player for The Yardbirds.
Ha! Ha! Said the Clown
"Ha!
 Ha!
 Said the Clown" is a song by Manfred Mann from 1967 which made No.4 on the UK Singles Chart (for 2 weeks), No.10 on the Australian Singles Chart, No.1 on the Austrian Singles Chart, No.1 on the West German singles chart, No.1 on the Netherlands Singles Chart, No.1 on the Norwegian Singles Chart, No.5 on the Swedish Singles Chart and when covered by Keith Relf of the Yardbirds made No.45 on the "Billboard" Hot 100. The Trevor Horn Band
The Trevor Horn Band are an English supergroup formed in 2006 as The Producers, consisting of record producers Trevor Horn (bass and vocals) and Steve Lipson (guitar), and musicians Lol Creme (guitar and vocals) and Ash Soan (drums).
 The band briefly adopted the name US before changing to Producers.
 Latterly, they have switched to the name The Trevor Horn Band. Armageddon (British band)
Armageddon were an English hard rock band formed in 1974.
 Their self-titled debut, "Armageddon", was recorded in England and released in the United States on A&M Records.
 The albums' original liner notes use the term "supergroup", as their personnel were drummer Bobby Caldwell (previously a member of Captain Beyond), singer Keith Relf (who had fronted the band Yardbirds and was a co-founder of Renaissance), guitarist Martin Pugh (from Steamhammer), and bassist Louis Cennamo (also formerly of Renaissance and Steamhammer). Armageddon (Armageddon album)
Armageddon was the only album released by British/American hard rock group Armageddon in 1975.
 It features vocalist Keith Relf of The Yardbirds and Renaissance; Martin Pugh, lead guitarist for Rod Stewart's "An Old Raincoat Won't Ever Let You Down" and Steamhammer; Bobby Caldwell, drummer for Captain Beyond and Johnny Winter; and Louis Cennamo who was the bass player and bandmate of Relf's in Renaissance and Pugh in Steamhammer. Made in Basing Street
Made in Basing Street is the first album by the English supergroup Producers, now known as The Trevor Horn Band, released on June 25, 2012.
 As the album's name suggests, it was mainly recorded in the legendary SARM Studios (formerly known as 'Basing Street Studios') in Notting Hill, which is now owned by producer and Producers member Trevor Horn, and was used to record such famous albums as "Led Zeppelin IV" and Queen's "News of the World". Mr. Zero (song)
Mr. Zero is a song by Yardbirds vocalist Keith Relf.
 It charted at #50 on the UK Singles Chart. The Yardbirds
The Yardbirds are an English rock band, formed in London in 1963.
 The band's core lineup featured vocalist and harmonica player Keith Relf, drummer Jim McCarty, rhythm guitarist/bassist Chris Dreja and bassist/producer Paul Samwell-Smith.
 They worked with several lead guitarists, launching the careers of Eric Clapton, Jimmy Page and Jeff Beck, all of whom ranked in the top five of "Rolling Stone" magazine's list of 100 greatest guitarists.
 The band had a string of hits throughout the mid-1960s, including "For Your Love", "Heart Full of Soul", "Shapes of Things" and "Over Under Sideways Down". Jane Relf
Jane Relf (born 7 March 1947) is a British singer and the younger sister of Keith Relf of the Yardbirds.