Hello
Sorry to hear of your sad predicament - as mentioned, this is a rare but known problem, see the Boxster forum.
I also know of someone who was on their way to Porsche Germany when theirs (996 4S) gave way in France. The car was trucked to Porsche in Germany, they looked at the fully stamped OPC service book and fitted a new engine FOC, which was very good service.
Try speaking to the dealer principal at your OPC citing these examples and asking if the same level of goodwill could be extended to you car, and if not, why not in writing.
If no joy, go higher to Porsche Cars GB and make it clear you will be contacting Porsche AG in Germany direct.
If that didn't work, I would place the car on my drive with a sign on the front stating that the engine was broken due to a fault and Porsche (and your local garage) would not help.[:'(]
Might get their attention, but I would recommend the softly softly reasonable approach first!![
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I hope it is resolved for you very soon.
Rob