I don't think the Series 836 signified any major differences from 810 series.
The only example was built originally as part of a cancelled TAA order for 816 series aircraft and subsequently designated a Series 836 when delivered as N40N to Union Carbide in the USA.
Likewise G-BGLC was part of the same cancelled order, but diverted to the Iranian Government as a Series 839.
Both aircraft had a similar subsequent history with, among other operators, the RAAF, Jet Air Australia, Oman Air Force and Royal Swazi National AW, before ending up with BMA and Air Bridge respectively.
G-BCZR was built as a Series 838 for Ghana Airways before going to Fields and then Dan-air.