Thank you both for your quick responses. I shall follow your suggestions. A compression tester is on order.

The fuel pump works - I get fuel flowing out of the fuel rail no problem when I disconnect the return pipe from the pressure regulator. 

Would you agree that the fact that the engine fires up and runs on 2 cylinders (and on the other 2) indicates that the injectors are delivering fuel and the spark plugs are providing a correctly timed spark? Even if not the precise quantity of fuel and the most perfectly timed spark should the engine not run on 4 cylinders if it runs fine on 2?

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