Arthur, you're a star.
As you suggested, the relays were the wrong way round!!!
The numbers and colours are as you say, and - stupidly - these relays fit in each others sockets.
The fans now run slowly, intermittently, from about 90 degrees and I'm hoping not to have to experience the "fast" fans at 100 degrees plus! However, I know they work as when I remove the plug from the sensor on the thermostat housing, they speed up after about one seconds delay.
Anyway, it's all sorted now, with the correct fuses.
All I need now is for the rain to stop and the sun to return in order to test it!!!
Can you fix that?
Thanks again. This is the first ever no-cost repair to my Mercedes Benz!!!