Thank goodness that I have now pretty much acclimatised to the high altitude, I spent nearly three weeks in La Paz at roughly 3600m above sea level, Copacobana which is roughly 3800m above sea level and now Cusco which isn't that different at roughly 3400m. I now feel absolutely fine with it, I must be fine when I did the hike in Isla del Sol, that was around 4000m above sea level.