XJS V12 fumes

Does it smell like exhaust fumes or more like raw gas?

For example, the XJS has those funny "S" shaped tailpipe extensions for a reason: to keep the exhaust fumes away from the center rear of the car, which is a vacuum area that sucks in fumes. The faster you go, the greater the vacuum. Bad weatherstrip around the rear decklid allows exhaust fumes into the car.

Look for oil gasket leakage near the exhaust manifolds.

If it smells like raw gas, then start hunting for wet-spot leaks.

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.