I'm almost at the stage of ordering Coilovers, what's the best way to "sit" the car so that I can measure up as there seems to be a lot differing views on the right sizes for the Haynes running mx5 with ssc rear end

Is there a step by step guide to checking what length shocks I need?

Set the car at ride height
The lower arms should be about level with the ground. Check ground clearance to sump ect.

Raise the front about 1 ½” and measure the length between the damper mounts. This is the open length

Lower the front so it is about 1 ½” below ride height and measure the length between the damper mounts. This is the closed length.

Repeat for the rear.

This will ensure you have adequate movement on the dampers.
In practice the springs and damper will constrain the movement to less than above so the dampers will not be topped out or fully closed.

Many thanks Dave just what I needed