For starters, don't use that "mechanic" again. If you have a clean and secure place to work, replace the valve spring yourself. Since one broke, you might want to replace them all. On many engines this can be done with the cylinder heads still on the car, not sure about the M117. See what the manual says.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.