A used Daimler V8 Reynolds timing chain tensioner, this came from a low mileage engine which had been rebuilt and hadn't done many miles before it threw a piston through the side of the block, normally you see quite a bit of wear on these items but this one must have been replaced when the engine was rebuilt.

SNG Barratts sell these for about £170 for a genuine JLR item and about £20 for an aftermarket one so make your own mind up on the quality of the aftermarket product and just to note the spring was slightly longer for the Daimler V8 tensioner, shorter for the rest of the XK Jaguar engines.