Replacing the fuel filter on your fuel injected XJ6/12 is an easy do-it-yourself service job. Remove the boot/trunk floor panel and, for easier access, the spare tire. The fuel filter is on the right side, well forward. It is a simple matter to loosen the clamps and mounting bracket, remove the old filter, and install the new filter, paying attention to the flow direction arrow on the side of the filter. BUT...
IMPORTANT - Be ready for some spilled fuel! Before removing the hoses lay down several rags or old towels to catch the dripping fuel and wipe the area thoroughly afterwards. Don't leave the keys in the igntion... just to avoid the odd chance of your kid brother trying to start the car while you have fuel hoses undone and, just to play it safe, have a fire extinguisher handy. I leave the old filter outdoors in a pan and allow it to drain and the fuel to evaporate. Don't store fuel soaked rags in your shop! Leave them outside to dry out.
Assuming clean fuel the filter should last 30,000 miles but, remember, a filter doesn't have an expected life. A clogged filter is not defective, it has done it's job. If you live in an area with known poor fuel, you'll probably have to change the filter much more often. Repeated clogged filters may also indicate that you are picking up rust and contamination from your tanks themselves, a somewhat common problem which requires tank flushing or prefessional cleaning to rectify. A good idea here is to avoid running the tanks below the 1/8 point or so.
The most common tell-tale sign of a clogged filter is low power, especially up hills or at wide throttle openings.
When available you can't go wrong with a genuine Jaguar replacement filter. However, some aftermarket alternatives include the AC Delco GF538, the Fram G3829, or the Lee LG8908. Many Jag owners prefer Bosch filters but I don't have that part number at hand.
Your fuel injected Series III Jag has umpteen fuel hoses and they really need regular inspection and attention. If orginal on your car, these hoses are anywhere from 8 to 21 years old and represent a genuine risk.
The best advice is to be safe and summarily replace all the fuel hoses, stem to stern. This will be a big job but your safety is the issue. This is a high pressure fuel system and a leaking, or worse yet, outright burst hose can spray fuel far and wide.
The under-bonnet hoses are particularly important as the high heat environment deteriorates them much more quickly than in other areas. The importance of this issue cannot be over-emphasized as engine fires are not unusual and almost always are the the result of leaking fuel hoses. Some owners replace the under-bonnet fuel hoses every five years - a good precaution. At very least you'll want to inspect fuel hoses several times a year and replace any hose which is remotely suspicious in appearance. You'll want to look for splits, cracking, and abraded areas.
When replacing hoses you'll need to specify "fuel injection" type hose as it is designed to withstand higher pressure than standard fuel hose. Hoses on the suction side of the fuel pump can be replaced with standard fuel hose as these do not carry high-pressure fuel.