According to sources the Luscombe was climbing out of its base airfield--the other aircraft apparently reported that it was an altitude of 1800 feet (presumably on QNH) when it was hit.
According to many people who were flying in the area the visibillity was reduced by haze.
Looking at the chart for the area, which is a busy piece of airspace with N/S transits, it has an ATZ, 2 or 3 private sites a glider site and a couple of microlight sites.
The FIR extends to FL045
Interesting set of circumstances dont you think?