We have a screened deck with a tin roof, Is there any way of insulating it to cut down on the heat. It is a large 4x13m area and it has 3 narrow
skylight sections in it. It receives morning sun so heats up early

Anything from simple Styrofoam to top of the line polyisocyanurate would help.

What are the rafters,2X4,2X6?

To install solid insulation to rafters I have cut it to fit,snug,then added strips of 1X to the edges to keep it tight to the sides(that allows for expansion of the rafters)

Any insulation would need to be covered at some point,say with soffit panels.

You still need to allow for the roof to breath, you don't want to trap condensation between the insulation and the underside of the roofing,1/2 - 1 inch is enough as long as the air can escape out the top.

If you want to keep the underside the same as it appears now,you could change the tin roof to a shingled roof,adding the insulation to the roof and OSB over the insulation,then the roofing on top, this would eliminate the ventilation problem below.