Example if I take a mini roundabout a bit quicker than I should I get a knock so hard you can feel it through the entire car in the seats and all. To say how powerful it is, when I showed it to an specialist E type mechanic he looked at me in horror and could only suggest the gearbox was out of alignment and moving. Checked it and it wasn't that.

After driving straight for a while the knock then repeats itself, whatever has moved moves back.
All the arms have been replaced as the old bushes had gone. I cant see any breaks on the front springs while they're in situ. The top mounts are crumbling and are due to be replaced this week but I cant see it explaining that unbelievably hard knock. Imagine someone hitting the bottom of a car with a sledgehammer , it is a brutal event.
I dont know what else to look for if the I find the springs are good after replacing the mounts and the replacement doesnt fix it.

Keeping my fingers crossed that someone here has experienced it and knows what it is!
Its a 2003 S type. Full recorded history from Jaguar (until I bought it) so it seems strange that it has an issue like this.

 

there is a certain device that has a few sound measuring probes. You place them in several locations on the car to find the source of the sound.
there is a scotty kilmer youtube video - finding noises in your car.

when you find the location, you wanna simulate the conditions on a elevator and try to see whats happening. Maybe a gopro or an action cam could help if you find the location of the sound

it could be anything at this point.
whitout more clues it's like schrodinger's cat

 

Try to avoid going to full lock, don't turn the wheel all the way if you can. Full lock puts the maximum pressure in the steering system. Does it leak?