Hi Mike & Kevin
Thank you both very much for the info. Firstly I stopped my subscription to Ancestry so I can't check the public members tree, although I think it was more than one tree giving the info that suggested Mary Ann Tyler's daughter Mary Ann Harrington married a Thomas Gilbey. There was even a photo of Mary Ann Harrington. So I can't check to see what the source was.
I had previously a note about Hannah Harrington and it certainly looks like it was her daughter who married Thomas Gilbey and therefore my previous research was correct and my Mary Ann Harrington was the daughter of James Harrington and Mary Ann Tyler.
I am now going to send for the birth certificate of Mary Ann Harrington born 16th October 1847 in Gosfield, just to confirm this. You are right about Ancestry trees, but with more than one tree suggesting I was wrong I thought I obviously was. Lesson learnt there eh.
Thanks again for your help and to everyone else. I'll post the result of the birth certificate, which I trust will show the mothers maiden name.
Dave