Thanks Arthur, that is exactly what I found.
I've replaced the blown 15A fuse on the blue (rearmost) relay with a 30A fuse and the fans now run properly - a minute or so after the A/C is switched on.
The Green relay - high speed/high temp - has a functioning 15A fuse which I will replace tomorrow with a 25A as you suggest. I don't want that fan blowing it's fuse in "emergency, super hot" situations. I suppose if everything is working correctly, this mode of fan operation will never be needed.
Whilst I was in the independent dealers buying a new pack of fuses, I splashed out and treated my E280 to some new repeater indicator lenses. Over here, our cars have to have repeater turn indicators towards the rear of the front wings (sorry...fenders?)about six inches in front of the door. These are orange, rectangular, about 2.5 x 1 inch high and are the only orange lenses on the car. I think this really cheapens the look of the car and my new white lenses (with "invisible" orange bulbs) look 100% better.
Anyway, thanks again from a very wet (and unseasonably) humid London!