I know how frustrating it is when you realise that your notes are not as accurate as you thought they were!! Many of us have been there. I have a note next to an entry for one of my lot where I have a date and next to it I have written 'How do I know this- prove it !!'
Probably the best way to make sure you get the right chap is to recheck your evidence ie birth, 1901 census, 1911 census, marriages if any, birth of children if any and any newer information you have and then you might be able to narrow down his death from the many Thomas Mitchells listed all over the country. Hope you find him.