Thanks everyone for all these suggestions! Really, really helpful :-) I'm going to write up a little list and next time i'm in Boots I'll have a better idea...
Just because it can be helpful to hear other people's thoughts, these are ones I've tried before and had no luck with:
Benefit They're Real: too spidery an effect for me, not 'natural' enough although I liked the liquid formula
Guerlain (in gold packaging, double ended): really expensive and just not good enough for the price - actually quite a decent mascara but not 'the one' and therefore not worth the 30 quid or whatever it was :-S
Maybelline pink and green one (someone mentioned this): actually quite good, and one of the cheapest ones, but it has quite a small brush which is quite fiddly for applying. A really good option if you're on a budget, but again - not 'the one'.
Clinique lengthening mascara (in a white tube): has a really strange brush that just really didn't work for me. I liked the idea but the standard Clinique dark green bottle one was way better. Again, an expensive mistake at £20 or so.
I also experimented buying just a cheapy clear mascara to see whether than would be enough and would look natural but also lift/separate my lashes - sadly it also made my eye lashes clump and just didn't look great!
Thanks again for all your ideas :-)