My attention then turned to their eight children. Five of these children I could trace from birth to death, one I knew had died and two I was not sure about. In this thread concerning the family I receieved a fantastic post from Nelmit who had looked up a poor house application. It gave an incredible amount of information that I had been looking for and alone to me it was a success story in itself. I won't go into it all here but the main point is that she gave a list of her children. The two chidren that I was having trouble finding were not listed even though they appeared in the Censuses with the rest of the family. I made the assumption they were dead but as their death fell in the OPR period I knew I would struggle to confirm actual dates.
The upshot was I had two brothers who I presumed to be dead by the time their mother made a poor house application in 1877 for the reasons given above. Hugh Marr had been born in 1826, appeared on the 1841 census and then never appeared again so I had a rough death period of between 1841 and 1851 for him. Thomas Marr was born around 1837, appeared in the 1841 and 1851 censuses before disappearing. Again I had another death period to look into.
Quite by chance a couple of weeks ago I was browsing through this forum and stumbled over a thread created by Stephan named Dunedin Deaths. As it had a searchable database I had a little play with it and searched for anybody with the surname Marr as I usually do. To my amazement I found the death of a girl named Mary Drummond Marr. The name was too coinncidental and had to be linked to my family somehow.
By a stroke of luck the Dunedin record showed she had been born in Scotland and using her death date with her age at the time of death I was able to work out her birth date. Luckily she had been born in 1866 so I quickly made a sprint for SP. To be disbelief her birth certificate showed her father as being Thomas Marr. From the birth certificate I found the date of her parents marriage and to by surprise Thomas was indeed the son of Hugh Marr and Mary Drummond.
Reverting back to the "City of Dunedin" site I was able to find Thomas had not died between 1851 to 1861 in Scotland as I'd thought for a long time but actually died in 1913 in New Zealand. It was an incredible find. Subsequent searches on SP have found he had actually married previously and his second wife had been married before too. The services offiver of the site was also helpful in acquiring additional info too when I asked.
It just goes to show that you should never assume anything however likely it looks. This was also nice too because I spend so much time and effort looking for ancestors and this site sort of found me.
.....(and a wee thank you to Stephan for his good sense of timing 
)