its like you say dave - all you have to do is drop the insert inside the strut tube - drop a ruler into the tube and measure the distance to the top of the tube - write that number down (this is how much the stut needs to be shortened by - then measure how much you want to cut off the top to keep the threaded section - i cut off around 30mm (to the inner sleeve step) - so then add both the numbers together - this is where you make the 1st cut - then cut off the threaded section (30mm) and re weld it back on - depending what you use to cut the tube - drop saw or pipe cutter - take into account the few mm of steel lost in those cuts.

you need to make sure that the re welded threaded section is aligned correctly otherwise u wont get the gland nut back on.

DIY run through here