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"Throughout the novel there are numerous references to the song "Old Clem." The first one occurs when Pip is pushing Miss Havisham around her room and she asks him to sing a song. Pip states, "It was a song that imitated the measure of beating upon iron" (104; ch. 12). The song comes from the blacksmith's patron Saint Clement. He is said to have an anvil as an emblem and was martyred by being tied to an anchor and thrown into the sea (Webber 28). Blacksmiths took a holiday on November 23rd, The Day of Saint Clement. It was often celebrated with loud explosions of gunpowder on anvils. A senior apprentice dressed in a cloak and mask to represent Old Clem and was carried in a procession that would move around town and stopped at all the taverns along the way. At all their stops the blacksmiths told the brief history of Old Clem and then passed a box around for donations. The money paid for the apprentices' supper. The story of Saint Clement has numerous origins, but all are relatively the same. King Alfred, who wished to build a palace, called all the craftsmen and announced that the man doing the best work would be proclaimed "Father of all craftsmen." As the palace was finished, the King held a banquet and all the craftsmen were to attend, each bringing a specimen of his work and the tools he used. All the laborers did such a good job that in order to avoid jealousy King Alfred gave the title to his tailor, who had made a coat of "surpassing beauty of colour and exquisite fashion." The blacksmith was terribly upset and did not return to his forge after the banquet. Because of this the mason and carpenter could not get new tools, and the warriors could not get new armor or weapons. King Alfred and all the craftsmen broke into the smithy and tried to do the work themselves, only to fail. St. Clement walked in and witnessed all the confusion and disaster in his forge and seemed to enjoy it. The frustrated king bowed to St. Clement and apologized and then held another banquet in his honor. This upset the tailor and during the banquet he crept under the table and snipped the apron of St. Clement, and to this day blacksmiths often have fringe on the bottom of their aprons, paying homage to Old Clem."

I am still looking for information regarding the provenance of the song, but I'm fairly sure there's nothing on the Net. Hard copy sources take a bit longer to access but watch this space.