It's been documented that the first tests of Laser on 729 were in fact at high power and the signal was the strongest ever heard from an offshore station but it was short lived as they didn't bother to use a tether line to hold the balloon in place so it fizzed and sparked and floated off after a short while. As for the choice of frequency, I don't know who made the decision but there were very few possibilities as the band was full of mainly fairly low powered local stations at the time as well as high powered networks. The brief high powered 729 tests must have caused big interference to the low powered Radio 4 relay in London at the time. There was also a navigation beacon at that time on or around 730 KHZ. 729 was eventually used for BBC local radio in Essex and strangely the beacon was closed down at around the same time due to interference (to BBC Essex). It's likely that nobody at Laser knew the offending navigation beacon was there but somebody must have got word of it.