@Боб - thanks very much for the detailed explanation. It seems I was 95% of the way to figuring it out myself, but couldn't bring myself to believe it didn't follow the nice neat rules in my grammar book!

Reading the other thread about много was very helpful - like Impulse I find myself wanting to understand the why, in the hope that if I understand it for one word then I'll understand it for all words. I think coming to terms with the fact that it's not all neat and perfect is one of the unexpected challenges of learning a new language. I simply have to accept that I have to learn a lot of grammar rules, and then also learn all the exceptions!

@lorente - thanks for your answer too. It demonstrates very well which is which

~ Lindsay