Wow, thank you everyone.
Ok, so... no it's not too hot. I learned the 'hand test' a while ago, and it's fine - corroborated, like I say, by a thermometer that is slightly more under the light than the plant is.
It's not wet - or rather it is, because I'd just watered her. I think if anything, she was too dry. Bit embarrassed about that one, but I've only been watering small amounts, which tonight's research suggests is a bad idea.
Riknstein..18 inches? Really? I'm an utter newb (but a sound engineer, so I know how the inverse square law works), but surely I'd lose too much light that way? No? Will you be mortally offended if I just go with 6 or so? Can I ask if anyone else has a similar problem with a 400W HPS at 5 or 6 inches (12-15cm)?
Thanks again for all the replies, guys, appreciate it.