The Christmas Puzzle

This is the eighth novel in the Pitkirtly Mystery series.
Through a random combination of circumstances, Jock McLean and Amaryllis find themselves playing Santa Claus and his elf respectively at the Pitkirtly Christmas Market, a blatant attempt to lure tourists away from Edinburgh. Proceedings are interrupted by a murder on Pitkirtly Island (not really an island) and by the activities of a new organisation called FOOP.
A sequence of events occurs which results not only in Christopher and Amaryllis having a picnic in the Cultural Centre but in Jock McLean buying new socks for the first time in living memory.