The living area gets extremely hot in the summer as it has a fairly high
glass area and gets masses of reflected light from the Estuary as well
as direct sun, so theoretically would insulating the rafters rather than
the ceiling make the loft hotter as more heat would get through the
ceiling?

Tri-iso insulation works in a different way to conventional insulation. In
practice, it means that it has some strengths and weaknesses in comparison.
One of its strengths is reflecting solar gain from the roof, which can be a
significant contribution to summer overheating.

Christian.