[AstralTraveller (David)]

You say they die but don't say how. Do they just wilt? Do they discolour then wilt? Do they grow at all between planting and dying?

My first guess is that you are burning the roots. Almost anything you put in to help the plants can burn them if it is too concentrated around the roots. In this case I'd be very suspicious of the clay breaker (calcium sulphate). That and the fertiliser need to be well and truly dug in and watered in before planting. Even leave it a while and get it rained on before planting. If there is too much concrete nearby the lime in the concrete can raise the pH too high, so I'd clear out as much as I could. Lastly, if the plants have become root-bound in the pot you need to trim off the deformed roots. The exact amount you remove depends a bit on the plant but you should leave only straight well-formed roots. If you don't the plant may survive but it will always be sickly. In servere cases, where you cut a lot of root away, you may need to trim back the top of the plant so the roots don't have too much foliage to support.

BTW a friend removed dozens of Eucs from his hobby farm and put in fresh seedlings after realising he had 'J-rooted' them. That means that he let a curve develop on the end of the tap root because the holes were too shallow. The trees would have grown but been very prone to falling over due to the poor root structure.