They were quite a jolly crowd inside

The Decline of the English Murder by George Orwell.

The next morning very early they turned me out of my cell to wash, gave me back my scarf, and took me out into the yard and put me in the Black Maria. Inside, the Black Maria was just like a French public lavatory, with a row of tiny locked compartments on either side, each just large enough to sit down in. People had scrawled their names, offences and the lengths of their sentences all over the walls of my compartment; also, several times, variants on this couplet –

Detective Smith knows how to gee;
Tell him he’s a cunt from me.

(‘Gee’ in this context means to act as an agent provocateur.) We drove round to various stations picking up about ten prisoners in all, until the Black Maria was quite full. They were quite a jolly crowd inside. The compartment doors were open at the top, for ventilation, so that you could reach across, and somebody had managed to smuggle matches in, and we all had a smoke. Presently we began singing, and, as it was near Christmas sang several carols. We drove up to Old Street Police Court singing –

Adeste, fideles, laeti triumphantes,
Adeste, adeste ad Bethlehem, etc.

which seemed to me rather inappropriate.

Also known as the Christmas carol, O Come All Ye Faithful.

O come, all ye faithful, joyful and triumphant!
O come ye, O come ye to Bethlehem;

From Wikipedia:

"O Come, All Ye Faithful" (originally written in Latin as Adeste Fideles) is a Christmas carol that has been attributed to various authors, including John Francis Wade (1711–1786), John Reading (1645–1692) and King John IV of Portugal (1604–1656), with the earliest manuscript of the hymn bearing his name, located in the library of the Ducal Palace of Vila Viçosa.

The original four verses of the hymn were extended to a total of eight, and these have been translated into many languages. The English translation of "O Come, All Ye Faithful" by the English Catholic priest Frederick Oakeley, written in 1841, is widespread in most English speaking countries.[2][4] The present harmonisation is from the English Hymnal (1906).

An original manuscript of the oldest known version, dating from 1751, is held by Stonyhurst College in Lancashire.