I've always used John Taylor storm covers and they've never let me down. Only just bought one for my S1. When I called him he asked if I wanted an extra piece to cover the vents on the bonnet as well.
That's right, Malcolm. He asked me about the small flap to cover the vents, and I had it fitted. When I was in Ireland in the rain in May that flap would have been very useful.
I also like the way the cover would keep showers at bay even if the hood has not been erected underneath.