Thanks again for all the replies.
My mother has sent a message to the match so hopefully she'll get back to us soon. However her tree on Ancestry only has her descendants and I think she herself is 88, born in 1930. (my Mum is 76). I have managed to construct a tree with what I hope is the right person. I've gone back to her great grandparents on her paternal side, however her mother's maiden name is Smith which is proving problematic, I'll have to get some certificates to prove it, so I need to wait for any contact with her before I do that.
We have got some rumours with regards to my unknown grandfather. My mum's older brother has some memories of that time but knew nothing of the secret. My mother's cousins did know the secret and has some memories. At the time of my mother's conception my nan was staying with friends in Nook, Wiltshire. They were from Portsmouth, so were avoiding the bombing. Nook has an army camp, which still exists to this day, but I have been unable to find out anything relevant as to who was based there in January 1941 which is when my mum was conceived. My uncle remembers by nan being particularly friendly with who he thinks might have been an Australian nicknamed Red, no proper name. He also seems to think that this gentleman may have died as a prisoner of war in a camp in the Far East. Bearing in mind that he would have been only 4-5 I'm not sure how accurate this could be. My mum's cousin also knew about this family secret, she overheard my nan telling her mum about it. She initially thought that the man was American, but as they hadn't entered the war by then it was more likely that he was Canadian.
This is all we have to go on I'm afraid, just rumour and conjecture which is why I'm really hoping that DNA is the answer.
Cheers,
Mark.