Ken Wallis, of Little Nellie fame, held the record for many years at around 17,500 feet but I think that has been beat in the USA and is somewhere around 22,000feet. When Brian Milton flew through the Alps on his world flight in a WeightShift Microlight he got through at 9,500feet. I phoned him after I had landed and he told me that it was a CAVOK day when he did it - if only!