It might be an environmental factor....seems to be receding a bit, in that there's no new brown spots on new growth, and it doesn't seem to be spreading at all....so perhaps it isn't fungal.
Beanz - Yep..I put tobacco in my j's and I smoke in here (it's the only place I can smoke that's away from the baby) - you should see my carbon filters....that's enough to make me quit...but thanks for the input...it might be TMV but looks like it isn't...
Hub - Fingers crossed they're going to grow out of it as you guys have suggested. Some leaves are looking weirdly shrivelled, so we're not out of the woods yet, but it's not quite as scary as I initially thought. I'll know more over the next couple of weeks as I get a chance to put them into flower.