I had the same issue with my 1999 XJ8, luckily enough I live in Essex near to Seb, QTrim, who has been recommended on here before. Takes a month or so get an appointment, but so worth it, did a fantastic job £350, worth every bean.
Being the proud owner of a 1999 XJ8 which is rust free at present. I have been considering a waxoil treatment for the underside to keep it clean. Is this something that anyone has tried and would you recommend it?
Regards
Paul