I have asked the WHOLE world and I should have probably started with Mumsnet.
It's a beautifully haunting arrangement of "in the bleak midwinter" - definitely not the Darke or Holst versions. I've also looked at ALL the other suggestions on Wikipedia and it's none of them.
I've (oh so slowly) put the tune of verse one onto clicky music software. Has anyone else heard of it?! I cannot find it ANYwhere but I know it exists because I sang it in my school choir (circa 2002)...
If anyone could help me it would make my Christmas!