Thanks for all your help,
Milliepede and
Galium!
I've finally had a moment to examine all the info and do some more research so here's a bit of an update:
I've identified Elizabeth Dawson (b. 14 Apr 1808, Margate) as the daughter of Thomas Dowson and Honor Ingle from the marriage mentioned. Elizabeth Dawson and William Ingle (her husband) were second cousins and this seems to confirm that the connection was through the Ingle family. Thomas Dowson had died before the 1851 census and I can't find him in 1841 so no info on him so far. They had six children I can find: John, Elizabeth, Thomas, Honora, James and Mary. Honor Ingle was born (according to the 1851 census) around 1779 in Bloomsbury - but I've had no luck finding her. She died in 1864 in Kent.
On to the Ingle side: Milliepede, where did you find this?
Timothy baptised 22 Nov 1781 parents Joseph and Elisabeth.
I can't seem to find that record anywhere - I have found parents that would fit. Joseph Ingle married Elizabeth Little in 1772 and they had six children: Thomas, Elizabeth, Joseph, Thomas Carter, Joseph, Joseph Robert. Obviously a high rate of infant mortality though I haven't found any death/burial records for the children.
Now: Hannah Martha Douce (Doucc). Galium, I think your baptism record is correct which means her parents were Thomas Dowse and Mary Cook who married in Deptford in 1760. They had six other childre: Mary, Thomas, William, William, Ann and Richard. I couldn't find a death record for Hannah: she could be the Hannah Maria Ingle who died in Lewisham in 1854, but I think it is more likely that she is Martha Hannah Ingle who died in Strand 1857 (which is where she was in the 1851 census).
Thanks again for the help - and if anyone has ideas on how to find more info on the connection between Timothy Ingle and Honor Ingle that would be fabulous! I'm also interested in finding out if the Douce/Dowse connection does connect up with the Dawson/Dowsons.
Cazo