I looked up P1646 in the JTIS DTC Summaries PDF file for 1999 XJ - Says Upper HO2 sensor on Bank A.
Bank A on US Jaguar models is the passenger side of the engine.
Bank A is equalivent of Bank 1.
There is confusion on Bank1 and Bank2 location on the engine - you look at the diagram below:
Here is a firing order diagram that shows bank 1 is on the left side of the engine viewed from the front of the vehicle (on US models it is passenger side).
__[ US models Bank 1 is passenger side)._________[ Firing order
__[ Firing order 1 : 5 : 4 : 2 : 6 : 3 : 7 : 8_________[ (Bank)-Cylinder (1)-1 : (2)-2 : (1)-7 : (1)-3 : (2)-4 : (1)-5 : (2)-6 : (2)-8
Also look at the thread on H2O heated sensor R&I - post# 1:
Recommends removing the front muffler assembly to gain access to the heated oxygen sensors.
The Front Muffler R&I PDF weblink is also in post# 1.