My name is Charley & my mum had a relationship 23 years ago with a man called peter Marshall, he worked at Coca Cola enterprises in Wakefield, then he moved to Hong Kong for work. My mum found out she was pregnant about 9 weeks after he had gone. She decided not to get in contact with him, because before he went to Hong Kong, he told her he was married & had a 2 year old daughter. Mum was really upset, but when she found out she was pregnant, she thought that she did the right thing by not telling him, as she didn't want to break his marriage up. She has regretted that decision as years have gone by. She doesn't know a lot about him, only that he lived in Sunderland, England, 23 years ago. I'm desperate to meet my dad before he dies. He doesn't even know about me. Can you please help me?

I'd start with the city of Sunderland, which has an online site. Find a newspaper there and put an ad in. Find out from your mother his age (even close) and if he has a middle name and what he did at Coca Cola, and any other little clues no matter how small.
Try Facebook, even though there will be many Peter Marshalls all over the world, and he might not be on it.
Take a trip to Wakefield (isn't that really far from Sunderland?) and hang out outside the Coke offices or plant and ask older looking people, and put signs on telephone poles. Same in Sunderland.