[zippy_gg]

I would suspect the latest work done on the car: new starter motor!
While I doubt the starter motor itself is the issue I think the solenoid or the wiring may be the cause of not turning over.
Hopefully the garage did not install anything Lucas! But I would definitely contact them and ask what was replaced exactly. Also is the "garage" a Porsche shop, or at least well versed in P-cars?
I am afraid you will have to crawl under the car to gain access to the starter... Try using a screwdriver to by-pass the solenoid and see if the engine turns over.