You need to take a moment and search for a better source for the rack. Here in the States, a steering rack for a 1996 XJ6 runs about $229 with rack exchange. That's about 128GBP. These racks come with a lifetime warranty so if you have to replace it in the future -- it is a swap of the old for a new one.
Stay away from Jag dealers and independent Jag repair people. When I did the rack on my '88 XJ40, it cost me $225 plus a $49.00 alignment fee. You should be able to get out of the whole thing for around 150GBP. (If you do it yourself)
You must first make sure it is not the lines that are leaking instead of the rack itself. These racks are famous for blowing the high pressure line
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