If Stephen married in 1840, he can't have been aged 18 in 1850....

Could this be him on the 1851 census? The surname looks like HASSILL, but he's an ex-soldier born in Essex.
HO107/2476 531 35 - Little Prospect Place, Pembroke DockStephen HASSILL - Head - 39 - Greenwich Pensioner - Felstead, Essex
Elizabeth HASSILL - Wife - 35 - Radnorshire
Mary A HASSILL - Dau - 10 - Scholar - Pembroke Dock
George HASSILL - Son - 9 - Scholar - Pembroke Dock
John HASSILL - Son - 5 - Pembroke Dock
Jane HASSILL - Dau - 2 - Pembroke Dock
Joseph HASSILL - Son - 1 - Pembroke Dock
Rambler