the national insurance office in newcastle has a department that offers a letter forwarding service to any person living in the United kingdom that is paying national insurance contributions or is in reciept of any form of benefit.
You can write a letter to the person you are looking for and on the envelope write their name at birth, their date of birth and any previous address or location you have for them, and anything else that would help identify them, like a mothers name for instance.
then you put the envelope in a bigger one and send it to the NI office , they will then forward the letter if the person is still living in the UK.
if not they return it to you. They do tell you if they have forwarded the letter but not where it went to, then it is up to the person you are looking for to contact you.
I only used this once for a lady and she was reunited with her mother after 58 years.
I no longer have the e4xact address and indeed, i am not 100% sure this system is still available, but try here first:
http://www.direct.gov.uk/Dl1/Directories/Usefu lContactsDetails/fs/en?CONTENT_ID=10011366&chk =OwKpu1
you could do an online search of the electoral roll for Glasgow or search the GRO index online (registrations of Birth marriage and death).