Hmm, things have gotten more complicated.
The death certificate for John Leach confirms he was a harness maker, which I know to be correct. It gives an address which, whilst the street is not one I've seen before, is in the area of Somers Town, St. Pancras and I know he moved around a lot within that area and Shoreditch. It gives his age as 52 meaning he was born 1814/15 (he died 09/03/1867) and this matches ages recorded on the 1841 and 1851 census. However, the only baptism I could find for him was in 1816 and gives his d.o.b. as 11/08/1816 (baptised 22/09/1816). It's quite possible the baptism is wrong but the church was where he got married which is why I thought it might be the one.
Anyway, I've just found his probate record. This confirms the date of death and the address that is on the death certificate. However, it says his wife is Elizabeth not Louisa. Louisa was still alive at the time of John's death and when she dies it says on her death certificate that she was the wife of John Foster, Harness Maker. I cannot find another marriage for John to an Elizabeth in a time frame that would match or one where his occupation matches.
What makes it more confusing is that one of his children, my 3xGG, was also called John Leach (lets call him Junior) and he too was a harness maker but he moved south of the river to Newington and later, Camberwell. His baptism says his parents were John and Louisa and that his father was a harness maker. His address is given as Rodney Street which is just up the road from Kings Cross.
So I know Junior lived near Kings Cross, his parents where John and Louisa and John was a harness maker. I've got John and Louisa on the 1841 and 1851 census with their 5 children all of whoms baptisms confirm the parents names and occupations and show the family moved around the Kings Cross area a fair bit.
The death certificate matches name, occupation, age (according to the census) but doesn't mention the wife's name. Just says that the informer is a Matilda Akers who isn't someone I know of in the tree. But the probate record, which matches the date and address of the death certificate, gives a different name for his wife?

I know that the name John Leach isn't rare and there would have been loads of harness makers living and working in that area but could there really be two John Leaches of the same age living in the same area, doing the same job but married to different people and only one, the one married to Louisa, shows up on any Census and yet both disappear from the 1861 census???
I can't work out which info is wrong. If the death certificate is wrong then why isn't there census, marriage or baptism records for john and Elizabeth. If the death certificate is correct then who is Elizabeth?