Wow thanks everyone for all the replies! You are definately right that with a name like Smith I will need to invest in some certificates. I have the marriage certificate for Alonzo's son John and that made me want to investigate the Smith family further.
The 1871 census that you are mentioning, are we concluding that this is the same George and Clara as the 1881, 1891 census records I found?
Also the marriage to Clara Witton that Jim1 found, is this more likely than the marriage to Clara Miller that I found? It would be interesting to see what year both these women were born to determine which one was more capable of having 18 children!
How do we explain George havin a wife named Leah in 1901? Could this still be Clara? Perhaps using a middle name? Or must it be a second marriage?
I had a look on the 1911 census and these are the only 4 George Smith's I found around the same age as my George living in Birmingham;
George 1 - aged 60 married to a woman called Eliza Maria 43 and had only been married for 1 year.
George 2- aged 60, widower working as a bedstead caster. Living as a lodger to Mary Baker, aged 71 widow born in London. She has living with her a daughther Rosina Hopson, her husband William Hopson and their children, Rosina Hopson aged 6 and William Hopson aged 1.
George 3 - aged 59 married to Sarah Jane Smith for 35 years. Working as a beerhouse keeper. Have 3 duaghters, Lily, May and Alice. Can rule this one out.
George 4 - listed as George Charles Smith, aged 59, single. Living with widowed sister Mary Ann Phillips aged 64 and her 2 children Reginald George and Annie Louise.
So unless George 2 or George 2 are my George it looked like her died before 1911.
I searched for Clara in 1911 and came up with 2.
Clara 1 - aged 61, widowed living in Aston. Living with 5 children, all in their thirties or late twenties - none of the names match my Clara's children.
Clara 2 - aged 60, single. Occupation 'private means'. Living with single sister Mary Ann Smith.
So it looks like there is no Clara in 1911 either, unless George had died and she had remarried.
I tried one last serach of the 1911 census for any of their younger children.
I found 1 Bertie Smith who would have been born in 1903, mine was born in about 1900 as he appears in the 1901 census. I'm guessing Bertie could be short for Albert and as the child go older he was called by his proper name, either that or he didn't make it to 1911.
I found 3 Leonard Smith's born in Aston around 1895.
Leonard 1 - born 1896 in Aston
Leonard 2 - born 1896 in Aston
Leonard 3 - born 1898 in Aston
I decided to focus on Leonard 1 and 2 and this is what I found.
Leonard 1 - parents are Harry and Minnie Smith so cannot be my Leonard.
Leonard 2 - parents are Henry and Anna Smith so cannot be my Leonard.
Frustrating!!