Wow, thanks for all your input. I hand't expected so much interest. Apologies for the typo regarding Margaret/Mary Buchanan. The 3 children of Charles Todd and Mary Buchanan are:
Margaret Buchanan born 22.4.1822 Lanarkshire, married Colonel Sir David Davidson on 2.9.1849 in Kelso, died 12.11.1899 Edinburgh.
Mary Buchanan born 1828/1829 (estimated from census records) Lanarkshire,married William Middlemass 27.111848 London, died 21.1.1927 Hampstead.
David Buchanan born 16.1.1830 Edinburgh, married Anna Wyndham Penruddocke 29.9.1853 Pewsey, died 30.5.1900 Rugby.
My g-grandparents were Godfrey Critchley-Salmonson and Caroline Middlemass. Caroline was born 26.12.1852 at Monkrigg, Haddington. She was the daughter of William Middlemass and Mary Buchanan. William was born 4.2.1808 at Dunbar ( the son of the Provost of Dunbar Christopher Middlemass)
George Dalziel WS appears to have dealt with all the legal matters of both the Middle mass and Todd families. He is an executer in all the family wills. The records of Cambridge University show David Buchanan as the ward of George Dalziel..
David and Anna had 11children - the first 4 (up to 1847) were born at Bragburgh Hall, Northants, the rest at Northfield House, Rugby.
I have not been able to find any of the birth or baptism records for Margaret, Mary or David - nor any 1841 census (apart from David who was at Rugby School in 1841).
Charles Todd had a number of siblings. One of these was a sister named Arthur! She appears to have acted as some sort of guardian. she was witness at their marriages, and in fact David's wedding certificate shows him living at her house Broomlands, Kelso. When Arthur died in 1862, she also left a considerable amount of money to her three neices, this time also naming their respective spouces in the will.
Hope this gives you something to go on. I have much more information which I will post later including more details of the Glasgow Herald article!