The chi rho is an early Christian symbol comprising of the letters XP of the Greek word ΧΡΙΣΤΟΣ (Christos). The chi rho symbol was adopted by Emperor Constantine as a symbol on his military standard. This chi rho can be found in Coventry Cathedral.
The building that this mosaic SP3379 : Mosaic detail in Cov Centre. decorates was originally the Locarno ballroom, hence the jolly design. The artist was Fred Millet.