192.com is OK, but only if they have married since 1984 or have a landline phone number which is listed with directory enquiries. I have traced many living relatives in connection with my family tree via
http://www.ukroll.com/ which allows you to buy access to the UK Electoral Register for just one day if you wish - still only works if they are on the electoral register but the majority of people are I find. Also - what about asking the letters page in your local newspaper to help by printing a short note - you can ask those who wish to reply to contact a PO Box number if you dont want to give out your address - The post office will advise you on how to do this. Do you know who they used to work for? Might be worth sending a letter c/o their last known firm to ask them to pass it on ...... if the employer was from a long time ago, ask them to forward the letter via the National Insurance office (in Newcastle I believe) by adding the NI No for the person to the envelope and sending it on .... you will need to supply a couple of spare envelopes and stamps but it might just work. Do hope you find them...... I know it sounds obvious but have you just "googled" them - in case they have any professional qualifications, special interests, charity connection etc, as this might throw something up.
Good luck.