Thanks for the advice ladies!
I'd checked out the sizing charts but don't really trust them, based as they are on height/weight and BMI rather than actual body measurements. According to the Skins sizing charts I should be wearing Large tights yet the Medium is what actually fits me. I think years of weight training has left me rather 'dense' and a lot heavier than I look, so anything based on BMI isn't reliable...
Ideally, I'd just try the bloody things on, but there aren't any shops locally who stock full ranges and I know from experience if I mail order them with the plan of returning them if they don't fit, I never get round to it. eBay is much cheaper and I've picked up several pairs of Skins tights brand new for under £20, so I think I may plump for the 2XU Mediums and see how they feel.
Thanks again 