Thanks everyone for the kind thoughts!
I've just bought Lightroom 5 and as a consequence I'm now shooting RAWs as well as JPEGs. I'm very glad I am because the JPEG of this image was overpowered (not surprisingly) in the central lightning area. The RAW allowed me to recover a lot of detail that would otherwise have been lost.
I used to hate the clouds but now I'm learning that they can be my friend too.... at times!
Cheers,
Mario