Thanks for the advice. I've just been down to my local dealer and bought a 120 cap. They looked at the 90 cap and proclaimed it non-original.
Now I just have to hope that the hoses on my car hold out at 1.2 bar, as opposed to 0.9!
Incidentally, I had the cylinder head gasket replaced on the car recently, and when the mechanic filled up the cooling system, he only put a little bit of anti-freeze in ... the reason being, "it is better to let the new gasket seal before full-strength anti-freeze is added." Mmmm. I'm not sure about that one, I might drain half the coolant and replace it with Mobil One anti-freeze.
1977 W123 280E auto
Cayenne Pepper orange
289,000km and goin' strong ... touch Zebrano.