Thanks Diddy
That was a great help. It looks as though Herbert Davidson was the father of Robert Davidson so I was able to go further back based on this info (Herbert Davidson is listed on a birth certificate for one of Robert's son's as a witness). Interestingly I've found that there is some real discrepancies with the spelling of Hebert and Davidson e.g Harbert, Harbour, Herby and Davis, Davies and Devis - all this for the same person according to some of the records I've checked on SP. One thing I did notice is that the Familysearch site has different birth dates to records on SP - is the data on familysearch generally reliable?
Again many thanks for your help
Jamie