The old XJS was notorious for fuel smell complaints, and although a bit
later in the run, the vintage you're describing was not immune from the
problem. It's caused by the continual expansion and contraction of the fuel
tank which fractures the internal baffle to tank skin welds. Tiny cracks
form at the baffle weld spots on the outside of the tank, which then permit
small amounts of fuel to leak out and  accumulate on the felt pad upon which
the tank sits. The only fix is to replace the tank with a later spec unit
and to replace the padding as well. I'm fairly certain this was subject to a
recall here in the States, not sure if it was Global or not.

This, folks, is one of the reasons Jaguar had to be bought by a parent who
knew 1) how to build cars, 2) when to admit there was a problem. This and a
host of other product issues went on for years and no one in Jaguar service
would either admit it was a  fault or initiate a fix.