Not 100% unfortunately - based solely on Thomas listing his place of birth as Cambridge on the 1851, and my belief that he is William's brother. Sadly thats as sure as I can get at the moment - desperately need more info basically!! Any other suggestions?
The Thomas Mottlee I have found and believe to be William's brother (but this is an assumption at this stage) was a Milkman on his marriage in 1838, and on the 1841 and 1851.
1841 he and his wife mary Ann, and a daughter Clara from Thomas's first marriage lived in Wardour St. I havn't determined who his first wife was.
1851 the couple are living at 5 Broadway, St Margarets, Westminster with Clara and a son Frederick (ref HO107 / 1480). They are indexed under Mottler.