Replacing the front shock absorbers and Vee mounts on my XJ40 seems to have cured the knocking from the front passenger side while going over bumps that's been troubling me for some time. To a casual inspection the Vee mounts looked OK, but levering hard with a big screwdriver while the front was in the air revealed the rubber cracking and separating. It looks like they were the originals.
The Vee mount replacement isn't in Haynes or the Jaguar service manuals that I've seen. There are brief descriptions of the process online, but I found this TSB for the X300 particularly useful. The Vee mounts and general design are the same as the XJ40, although the TSB describes lots of steps that the DIY descriptions didn't mention. I found it possible to skip steps: 3-6, 8, 9, 12, 21-25,
27, 28.
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The most difficult part was getting a spanner onto the head of some bolts next to the rubber which had bulged out against them. One in particular took ages trying to lever the closed end of a spanner on using a screwdriver since I couldn't get a good angle to use the open end. You also need to lever the new Vee mount spigots into their holes while the subframe is lifted to just the right height. You need the handle of the jack lifting the subframe accessible while looking through a wheel well to see how well a front Vee mount spigot is aligned as you lift it up.
Other info:
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It took me about three days to replace both Vee mounts and the front shock absorbers (which are disconnected to replace the Vee mounts anyway, and allow better access if removed), which is about my typical speed for a job others say takes around 2-3 hours. I bought the new Vee mounts from SNG - unfortunately the cheap ones people in old forum threads mention buying from RockAuto seem to be permanently out of stock now.