@Mark if I can be if any help at all to you, don't hesitate to get in touch.
By the way, that Norse / Gaelicised then Angliscised placename is spelled Carbost. In Lewis the same name is found at 'Garrabost' at An Rubha, east of Steornabhagh, and that form is a bit more phonetic, more of a help with a guide to the Gaelic pronunciation, not that it is really Gaelic anyway.
(In Gaelic various combinations of adjacent consonants trigger something called svarabhakti or an epenthetic vowel, a vowel which appears and separates the consonants and its quality is a repeat of the vowel before the first consonant. The r+b in Carbost is an example of this rule being triggered so it is pronounced arrab.)
However someone may be pronouncing it totally under the influence of English, mistaking it for an English word, like carboot sale in which the r is not pronounced at all in English English.