I paid 45 p - 55p (never 56p) in 1974 depending on where I shopped and depending on time of year (given high inflation at the time). List price could have been 48p or 56p in 1973 but I'd go for 48p
I found a blog where the contributor said he paid 45p (in instalments to his mother!) for Merry Christmas Everybody in 1973. Unfortunately that's halfway between 42p and 48p but as prices rarely go down, I'd be more inclined to opt for 42p!