A potential source of records regarding the surname Caldwell in the 12th century is that if the Catholic Church. The original records woud be in Latin, but translations into English might be available. They might be in the papal library in Rome, or in any of several Archdioceses. The Catholic Church Archdiocese of Glasgow encompasses the Counties of Lanark, Dumbarton, and Renfrewshire, part of Ayrshire north of Lugton Water, the district of Baldernock in Stirlingshire, and the Cumbrae Isles. The See was founded between 540 and 560 by St. Kentigern, or Mungo, who died January 13, 601. The first bishop of the restored See was John Eochy, or Achaius, who held it from 1115 till 1147. He had twenty-three successors in actual possession till 1560, when an Act of the Scottish Parliament prohited the Catholic faith. On January, 9, 1492, Innocent VIII raised the See to metropolitan rank, attaching to it the suffragan dioceses of Argyle, Dumblane, Dunkeld, and Galloway. James Beaton, nephew of the celebrated cardinal of the same surname, was the fourth and last archbishop of the old hierarchy. In 1560, eight years after his nomination, he was forced to retire to France, where he acted as confidential agent of Queen Mary, and later openly as ambassador for James VI, till his death in Paris, 25 April, 1603. He carried away with him the diocesan records, two of which deserve special mention: (1) "Registrumn Vetus Ecclesiae Cathedralis Glasguensis", in handwriting of the twelfth and thirteenth centuries, and (2) "Liber Ruber Ecclesiae Glasguensis", with entries from about 1400 to 1476. These, along with other records, were in 1843 printed in a volume for the Maitland Club under the title: "Registrum Episcopatus Glasguensis: Munimenta Ecclesiae Metropolitanae Glasguensis a sede restauratxxxxxx saeculo ineunte XII ad reformatam religionem". Perhaps copies might be available at one of the universties in your country.

Before the Reformation, all of the parishes of Renfrewshire were included within the deanery of Rutherglen, one of the ten deaneries of the diocese of Glasgow. After the reformation, the Policy of Presbyteries and Synods were introduced. In 1590, almost all of the parishes of Renfrewshire were formed into a Presbytery with its seat at Paisley, and remain so to this date. The presbyteries of Paisley and Glasgow constituted the Synod of Glasgow, which joined with the Synod of Ayr in 1639 to form the united Synod of Glasgow and Ayr.
Perhaps one of our readers can shine more light on this.