That Middlesex 1841 census entry sounds right. Though the absence of a child younger than Jane is a bit of a puzzle. It would really be good to see the microfiche; perhaps Mary Ann was 1 month old rather than 1 year and, in the panic of emigrating, wasn't registered (though the variable spellings of HEASMAN might just mean that we haven't picked her up).
As neither William nor Jane was born in Middlesex, might this be Jane:
Jane Elizth AYLMORE, parents Josiah and Ann, bap 5 Aug 1818, Saint Thomas, Portsmouth, Hampshire.
Josiah and Ann had (at least):
Jane Elizth. AYLMORE, bap 5 Aug 1818, St Thomas, Portsmouth
Amelia AYLMORE, bap 2 Sep 1821, St Mary's, Portsea, Hampshire
Thomas Reeves AYLMORE, bap 10 Feb 1823, St John's, Portsea
Sorry for the misunderstanding Lu, I wasn't offering to spend my money

- just happily suggesting that you download more certs i.e. that you spend more of
your money!

The 1836 OPR marriage entry for William & Jane just might have some more clues - but, of course, might have nothing more than is in the IGI.
Cheers,
JAP