Thanks, Tim for your interesting and very helpful ideas.

Yes, Ann Wes(t)cott is Elizabeth's mother. She emigrated with the family to South Australia, aged about 60. She was in the almshouse in 1851 so it would have been a blessing for her I imagine.

Elizabeth, however, was baptised Elizabeth Wescott as the illegitimate daughter of Ann Wescott (not Stanlake) in 1810 (found in Bishop's transcripts for Mary Major, Exeter via Family Search).
Unless Ann gave the name Wescott in anticipation of marriage?

There is no sign of James Westcott in the 1841 census. But he could have died or left.

And there are also young Helen (16 years younger than Elizabeth) and Samuel Westcott (23 years younger) in the 1841 household with Ann . I have no idea how they fit in. And can't find them in later censuses.

I found a baptism for a Samuel to Elizabeth and James Westcott: 1831 in Parish of Charles, Plymouth. That could be him, if, as you suggest, Elizabeth Ann is her full name. This James was a Cordwainer.
The children are a long way apart in ages.

The two younger ones did not go to South Australia with the rest in 1855, but they were aged 31 and 24 by then.

Yes, I can see that your idea fits.
Any further thoughts?
thank you