If the leak is from the ‘O’ rings for the oil cooler feed and return pipes, unless you operate in high ambient temperatures, the oil cooler is surplus to requirements and you can remove the adaptor and bolt the oil filter housing directly on to the block. Many searchable posts in the XJ40 archives on this subject - here are a couple of examples :-
The only picture I can find of the oil filter attachment is for the later models with the oil cooler bypass feature item 4 (when they deleted the cooler) but the arrangement is the same on the earlier models with an oil cooler. You can simply remove item 1 and bolt item 8 directly to the block using shorter bolts.