Hi, I am also researching Hasler in Essex (there's a lot of them!). I have a Martha Jane Hasler b1831 and her brother James b1821 they are on 1861 census visiting Stock family at North End, Great Waltham. Martha and James' parents seem to be William and Susannah as per 1851 census and they also had another daughter Anna b1840 . I note that another researcher has in their tree info, that William's parents were George b.1739 died 1821 and Martha Gurder 1745-1817. William and family were dressmaker/tailors according to both census info. I'm trying to find the connection between James Stock's mother Mary who married a Hasler, after her husband (also James Stocks) died but I don't have his first name.
I did find a Mary Stock who married a John Heasler at Great Waltham 1806, but hard to know if it is my relative or not.
Hope this is of some use/interest to you.